g4bl path should point to third party
self.path_g4bl = os.path.join(os.environ['MAUS_ROOT_DIR'], \ 'third_party', 'build', 'G4beamline-2.12-source', 'bin')
fails if MAUS was built with maus_third_party set to a different location.
In that case, MAUS_ROOT_DIR and MAUS_THIRD_PARTY are not the same, and the third party g4bl can't be found in MAUS_ROOT_DIR
Since g4bl is a third party anyway, the above search should look for it in the MAUS_THIRD_PARTY path