Attribute handling, error handling, Tables, Folders

[yhteoh]

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Features
  Includes Support for table and document data #10

• What is the current behavior? (You can also link to an open issue here)

• Cannot set attributes for Datastore
• No errors for invalid GroupBase
• Datastore groups validated with model validator
• No schema for groups stored in hdf5
• No protection for special attributes
• No support for constrained datasets, optional datasets and dictionary of datasets.
• bidict for dtype mapping
• No flexible shaping support on dataset

• What is the new behavior (if this is a feature change)?

• Datastore has attrs field
• Renamed stored model signature in hdf5 file from model to _datastore_signature
• GroupBase only supports additional fields of type Dataset, attrs of type Attrs and automatic determination of class_
• Support Optional[Dataset] and Dict[str,Dataset] in Group
• Json schema of groups stored when dumped to hdf5 as an attribute _group_schema
• Protection for special attributes (_datastore_signature, _group_schema)
• use enum instead of bidict for dtype mapping
• refactor validation of Datastore and Group
• Added constrained dataset (condataset)
• Added CastDataset enabling automated casting of numpy arrays to a Dataset
• Dataset can be flexibly shaped when initialized without data, when data is assigned the shape is checked for compatibility and turned into a concrete shape.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  No

• Other information:
  Partially fulfills Features to add #6