BUG: Still more bugs in freesurfer.
ENH: FS52 longitudinal processing does not yet work.
ENH: Clean up labels maps with prior probabilities.

Trying to find the right morphological operators
to make nice clean looking segmentations maps.
ENH: Fix T1-only segmentation processing.
ENH:  Explicitly check on Freesurfer pathing and prohibit environmental freesurfer paths.
STYLE:  Make variables more robust for Freesurfer running.

The subjectid by freesurfer is really a sessionid, so make this more clear in the code.
Also added better directory handeling for error results.