Add dat files in DSIDs and split handling of commong from DSID dirs

Changes

• Split handling of DSID and common directories (since different rules apply)
• Allow to add *.dat files directly under a DSID directory but not in DSID/MadGraphControl/*.dat
• Do not allow common/*.dat (it is possible though to have a dat file under common/MadGraphControl/dat/*)

Tests

https://gitlab.cern.ch/atlas-physics/pmg/mcjoboptions/-/jobs/7205968

Closes #76 (closed)