documentation of data.py #48

Since we are training a number of models with HCSDataModule, it is timely to document data.py to clarify the flow of data through different methods.

I suggest the following:

• Add a note to HCSDataModule docstring that explains how the data on disk is turned into a batch (what public and private methods are called in the process).
• Add docstrings to all of the above methods.

@ziw-liu additional improvements are welcome, but the above should be sufficient to understand the design.