partial #281 refactor of NIFTIFolderDataset

In this branch:

• fix: error with variables (class => object)
• fix: error in __getitem__ when applying target_transform
• rename module file + rename external TorchTabularDataset for naming coherence
• improve typing of methods
• improve: make variables private when reasonable
• improve: handle exceptions with fedbiomed conventions
• add getters needed for properly testing (and for reasonable use ...)
• improve unit tests
• improve docstrings
• improve imports style
• not an abstract class

Remains to be done for completing #281 (closed):

• more error tests (try/except) for errors, parameters type + corresponding unit tests