Training of a nn-Unet-v2

[plbenveniste]

• Created the convert_bids_to_nnunet.py file in order to have something specific to the zurich_mouse dataset.
• Created the split_dict.json to select the files from the dataset for training, validation and testing.
• Created a notebook to work on the training of a nn-Unet-v2 model (to be converted into a .py file afterwards)

Related to #30, #38
Fixes #40, #39

[plbenveniste]

This branch contains the following python files:

• utils/convert_bids_to_nnunet.py
• utils/convert_nnunet_to_bids.py
• utils/crop_image_and_mask.py
• utils/extract_slices.py
• test.py
  Each file has formatted in the correct way.

The README.md file has been updated to describe the project process.