[ENH] Implement XNAT Extract DAG

This DAG would carry out pulling data from our central XNAT repository and pull data into the filesystem. It would run dm_xnat_extract.py with appropriate configuration being pulled from datman.config. In addition given that there are plans to implement a BIDS sub-command within the dm_xnat_extract.py script - the DAG should also read the config to determine whether to run XNAT extraction using dcm2bids or using the standard dcm2niix utility.

[ ] Merge PR from https://github.com/TIGRLab/datman/pull/317 implementing dcm2bids
[ ] Merge PR from https://github.com/TIGRLab/datman/pull/318 implementing a configurable defacing step to the data on extraction
[ ] Write a DAG definition file for the XNAT extraction and initial data pre-processing step (i.e defacing).
[ ] Add dcm2bids into setup.cfg as an optional dependency should users choose to use BIDS

TIGRLab / air-tigrs

[ENH] Implement XNAT Extract DAG #11