Support inhomogenous element blocks (in input only)

[rjoussen]

Description and Context

Motivation: Most mesh generators do not offer a simple interface to create strictly homogeneous meshes, e.g. when transitioning from a tet to a hex region or for unstructured hex/quad-dominated meshes. 4C so far requires strictly homogeneous mesh blocks, i.e., one cell type per block, which often requires post-processing such meshes with meshio or something.

This PR add support for inhomogenous mesh blocks. External meshes with inhomogenous blocks (only vtu and gmsh) are no longer rejected, but their inhomogenous blocks are internally converted to homogenous blocks, which all share the same group_id and (if specified) group_name. The internal id used for element blocks so far (which needed to be unique) is removed.

An inhomogenous external mesh block can be referenced in the input file as follows:

STRUCTURE GEOMETRY:
  FILE: solid_gmsh_mixed_element_block.msh
  ELEMENT_BLOCKS:
    - NAME: MixedVolume
      SOLID:
        HEX8:
          MAT: 1
          KINEM: nonlinear
        TET4:
          MAT: 1
          KINEM: nonlinear
        ...

The user must ensure that they define every cell type present in the external block, otherwise an error is thrown. We then pick the right definition for each cell type. Currently, it is possible to define more cell types than present in the external block. The additional cell types are simply ignored. The idea behind this is that it will make it much easier to switch between different meshes by just defining all the cell types your mesh could have. Providing no cell type definition at all throws an error.

The grid generator retains its old mode where only one cell type is allowed.

Note: this is not a breaking change, since specifying only one cell type per element block works perfectly fine still.

AI usage:

Parts of the code AI assisted with:

• replacing filter_by_cell_block_id by filter_by_cell_block
• writing 4C_io_mesh_test_utils_test.hpp
• finding all occurrences where we still rely on the internal ID of an element block (not the new group ID)

[Copilot]

Pull request overview

This PR extends the mesh input pipeline to accept inhomogeneous external element blocks (mixed cell types) for VTU and Gmsh by splitting them into homogeneous internal CellBlocks that share a common logical group_id / optional group_name, and updates downstream mesh filtering, element creation, and tests accordingly.

Changes:

• Update VTU/Gmsh/Exodus mesh readers and MeshInput data structures to support multiple homogeneous cell blocks per logical group (group_id).
• Update mesh reader input parsing to select logical blocks (by ID/NAME) and apply per-cell-type element definitions within a logical group.
• Add regression tests and new test meshes/input files for mixed-cell logical blocks (VTU + Gmsh), plus test utilities.

Reviewed changes

Copilot reviewed 25 out of 25 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/list_of_tests.cmake	Registers new integration tests for mixed element blocks (VTU/Gmsh).
tests/input_files/vtu/solid_vtu_mixed_element_block.vtu	Adds VTU mesh fixture containing mixed cell types in a shared logical block id.
tests/input_files/solid_vtu_mixed_element_block.4C.yaml	Adds 4C input using per-cell-type definitions within one referenced block id.
tests/input_files/solid_gmsh_mixed_element_block.msh	Adds Gmsh mesh fixture with a mixed physical group.
tests/input_files/solid_gmsh_mixed_element_block.4C.yaml	Adds 4C input referencing a mixed physical group by name with per-cell-type definitions.
src/core/io/tests/test_files/vtu/mixed_hex8_tet4_same_block.vtu	Adds small VTU unit-test mesh for mixed hex/tet in same block id.
src/core/io/tests/test_files/gmsh/mixed_hex8_tet4.msh.test	Adds small Gmsh unit-test mesh for mixed hex/tet in one physical group.
src/core/io/tests/4C_io_vtu_test.cpp	Updates VTU tests to use group-id lookup helpers and adds mixed-block splitting test.
src/core/io/tests/4C_io_mesh_test.cpp	Updates mesh filtering test to use new filter_by_cell_blocks API.
src/core/io/tests/4C_io_mesh_test_utils_test.hpp	Adds test helpers for retrieving cell blocks by group id/name and by (group id, cell type).
src/core/io/tests/4C_io_gmsh_test.cpp	Updates Gmsh tests to use group-id lookup helpers and adds mixed-group splitting test.
src/core/io/tests/4C_io_exodus_test.cpp	Updates Exodus tests to reflect group id/name semantics.
src/core/io/src/4C_io_vtu_reader.hpp	Updates VTU reader doc to remove “homogeneous-only” constraint for block_id.
src/core/io/src/4C_io_vtu_reader.cpp	Splits VTU logical block ids into multiple homogeneous internal cell blocks keyed by (group id, cell type).
src/core/io/src/4C_io_meshreader.cpp	Updates element-block selection and element creation to handle multiple cell blocks per logical block and per-cell-type definitions.
src/core/io/src/4C_io_mesh.hpp	Refactors RawMesh::cell_blocks to a vector, introduces group_id/group_name, updates CellBlockReference, and adds filter_by_cell_blocks.
src/core/io/src/4C_io_mesh.cpp	Updates lookup tables and filtering logic to operate on cell-block pointers / group ids.
src/core/io/src/4C_io_gridgenerator.cpp	Keeps grid generator restricted to one cell type group via input spec and adds a runtime assertion.
src/core/io/src/4C_io_gmsh_reader.hpp	Updates Gmsh reader docs to reflect mixed-cell physical group handling.
src/core/io/src/4C_io_gmsh_reader.cpp	Emits one homogeneous internal cell block per element type while preserving the physical-group tag/name as the logical group.
src/core/io/src/4C_io_exodus.cpp	Updates Exodus reader to populate group_id and group_name and uses vector-based storage.
src/core/fem/src/general/element/4C_fem_general_element_definition.hpp	Extends element definition parsing/spec to support multiple cell-type groups (map keyed by CellType).
src/core/fem/src/general/element/4C_fem_general_element_definition.cpp	Implements multi-cell-type unpacking and configurable one-of vs all-of grouping in the input spec.
src/core/fem/src/discretization/4C_fem_discretization.cpp	Updates assertion message to reference element block label/cell type (but currently contains a small typo).
src/core/fem/src/discretization/4C_fem_discretization_builder.cpp	Updates auto-clustered block creation to populate group id/name with new CellBlock constructor.