ENH: Update PETsurfer module

[mnoergaard]

Summary

The PETsurfer module in Freesurfer (centered around the functions mri_gtmpvc, gtmseg and mri_glmfit) needs minor updates to make the newest implementations to be in line with nipype. This includes the addition of a few flags to be added to some of the functions, but also some of the main classes need a renaming to be fully consistent with the function (for example LoganRef should be renamed to Logan). I will go ahead a draft a PR that solves these issues.