Enh/afni4d

Hi Chris,

Here's the feature-complete patch for afni4d support.  I clarified the afni parsing functions, and a considerable refactor to the various overriden formset save() functions was necessary to squash all the bugs.  It should be in good shape now.

I'll still add a few small things, such as the conversion script for existing sessions.

Oddly, the commits from the other pycortex PR are also in this one...I know you haven't merged it yet, but that still seems odd...

Cheers,
Gabriel