Factorize get subject session

[maximemulder]

This PR (still a draft) extracts the "get subject session" operation out of BasePipeline, with the goal of making it easily reusable by all scripts, including those that do not use BasePipeline (new DICOM archive and eventually BIDS import). It also modernizes the database access by using the new SQLAlchemy abstractions.

This PR should eventually be split into several PRs to make it easier to review (sorted by dependency):

• New SQLAlchemy abstractions
• Move remaining subject validation logic into validate_subject_ids
• Extract get_subject_session in its own file

Depends on:

• Improve subject configuration structure #1150

Is depended on by:

• New dicom archive #1117 (to implement Populate SessionID in mri_upload and tarchive tables at the time of insertion #1044)