Refact Dataset, Dataloader and DataModule

[FilippoOlivo]

Description

Refactoring of Dataset, DataLoader and DataModule to improve maintainability, flexibility and scalability. Specifically the following restructuring will be applied:

• Refactoring Condition:
  • Restructure inheritance
  • Add __getitem__ method
  • Add create_dataloaders method
  • Add __len__ method
  • Remove "data attributes" and add a dictionary self.data
  • (Optional) add compute_residual function (maybe in a later PR)
• Dataset: create a class which manage subset of conditions (contains the condition and the indices)
• Datamodule
  • Create _Creator class (store Dataloaders)
  • Create _Aggregator class (iterable class which is passed when a dataloader is requested by Lightning)

Checklist

• Code follows the project’s Code Style Guidelines
• Tests have been added or updated
• Documentation has been updated if necessary
• Pull request is linked to an open issue