Add support for data in BIDS format

[p-j-smith]

Fixes #13
Fixes #23

• modify the analysis script to handle data formatted according to BIDS, including handling subjects with multiple sessions
• save the session-level outputs to the derivatives folder of each session
• remove /code directory - all outputs are stored directly in the derivatives/enigma-pd-wml directory of each session
• create a top-level zip file (enigma-pd-wml-results.zip) that contains all session-level zip files. This zip can be shared with the ENIGMA-PD Vasc team
• update docs to reflect changes:
  • also remove mention of MRIcroGL for DICOM to NIFTI conversion. Instead refer to Nipoppy and https://github.com/ENIGMA-PD/FS7?tab=readme-ov-file#getting-started for converting data to BIDS format

These changes simplify the command for running the container has been simplified (if running from the top-level BIDS directory):

docker run "$(pwd)":/data hamiedaharoon24/enigma-pd-wml