Check environment and run FreeSurferEnv.sh if necessary

[effigies]

Addresses #102.

Check os.environ and run $FREESURFER_HOME/FreeSurferEnv.sh if not the most recent script to edit the PATH. Parse the resulting environment and pass to mri_vol2surf, avoiding modifying the base process environment.

[larsoner]

LGTM. I assume you've tried it and it works?

[effigies]

Yes, run through all new paths.

[effigies]

Though, it would mistake a manual PATH="$FREESURFER/bin:$PATH" for having previously run FreeSurferEnv.sh. If that's a problem, I could use a different check.

A possibility, since mri_vol2surf depends on $SUBJECTS_DIR:

if not (env['PATH'].startswith(os.path.join(env['FREESURFER_HOME'], 'bin'))
        and 'SUBJECTS_DIR' in env):

[larsoner]

I think that's an okay corner case to address later if need be

[coveralls]

Coverage decreased (-0.25%) to 72.31% when pulling b70372def668090a12cbe26c2d2a73af9e36e253 on effigies:freesurfer_env into bcdd7a08ccc56dec9272b211922cb09d7247b4f9 on nipy:master.

[agramfort]

if this works I am happy

[mwaskom]

This won't work on windows, but I am guessing that is not an issue because this whole function won't work on windows anyway?

[effigies]

That was my assumption.

[larsoner]

Thanks @effigies!