Feature: Add PPCG iterative diagonalization solver #15838

Job	Run time
Build without ELPA	9m 1s
Build without LCAO	5m 48s
Build without LCAO and MPI	4m 22s
Build with CUDA support	16m 55s
Build extra components with Intel toolchain	18m 43s
Build with Intel toolchain	8m 7s
Build with GNU toolchain	9m 22s
Build without MPI	7m 5s
Build extra components with GNU toolchain	24m 2s
	1h 43m 25s

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