Add a feature to use the fasttrack2s3.py data types to detect modalities present in a session

[ericearl]

It would be useful to know when a session has anat, dwi, fmap, and/or func data. This way different tactics could be selectively deployed in pipeline.py and bids_corrections.py. For instance, when converting only-dwi you don't need the workaround in pipeline.py and it could be disabled. Or, again if converting only-dwi, you could skip trying to apply corrections meant for anat, func, and fmap data.

[ericearl]

Passing the same abcd-fasttrack2s3.py -d argument(s) and abstracting out the DATATYPES dictionary would make this a solvable problem. See #33 for the abstracting out DATATYPES issue.