BUG: Fixed typo in singleSession for T2 passing to inputSpec of segmentation workflow
ENH: Added commandline option to use T1 as sentinal file (off by default)
ENH: Add logic for better skipping

It is not necessary to run jobs if all the necessary outputs
already exists.  This patch does better verification based
on the requested inputs.
ENH: Add new Mattes Mutual Information implementation.