Add CGS parcellations

[oesteban]

A request from @yasseraleman, which requires some heavy lifting but I believe will push TemplateFlow a long way forward.

GGS are the surface counterpart of FreeSurfer's GCAs. This is an in-house format for probabilistic segmentations/parcellations (GCA = Gaussian Classifier Atlas).

We've had some discussions at two fronts:

• FreeSurfer's mailing list
• GIFTI-dev forum

TL;DR the first link just had one answer from D. Greve that we can look into the C source code to try an understand the format (which @effigies had already scouted). The second link contains some discussion regarding about how to write this into GIFTI, and it looks like the answer is not immediate (I kind of side with Rick in all his comments).

In order to make this happen, we will require:

1. Adding support for CGS (and probably GCA as we are at it) into nibabel
2. Figure out the right CIFTI intent and suffix.

Both seem doable from the outskirts. However, I don't think we can assign this issue a high priority.