MRIsCALabel requires full path names, ignores subjects_dir

[ariekahn]

Summary

MRIsCALabel (and presumably a number of other tools) have their inputspecs set up with exists=True for a number of arguments that can be inside the subjects directory

Actual behavior

MRIsCALabel(
...
canonsurf="rh.sphere.reg"
...
)

TraitError: The 'canonsurf' trait of a MRIsCALabelInputSpec instance must be an existing file name, but a value of 'rh.sphere.reg' <type 'str'> was specified.

Expected behavior

mris_ca_label should be passed the subject_id along with 'rh.sphere.reg', which it correctly interprets as a [subjects_dir]/[subject]/surf/rh.sphere.reg

Is this intended behavior?