Add support for fsaverage

[kahinimehta]

• Data with fsaverage CIFTI derivatives
• Add fsaverage to allowed spaces
• Run on the test data and debug from there

[tsalo]

The CIFTI connectivity workflow does not have any transforms implemented, so we would need the same transform resolution steps that the NIFTI workflow has in order to support any CIFTI spaces other than fsLR.

[tsalo]

From BIDS:

The fsaverage is a dual template providing both volumetric and surface coordinates references. The volumetric template corresponds to a FreeSurfer variant of MNI305 space. The fsaverage atlas also defines a surface reference system (formerly described as fsaverage[3|4|5|6|sym]).

I am not sure what the MNI305 FreeSurfer variant is, unfortunately. I think we would need to be able to transform the atlases to this space.

[mattcieslak]

I assumed the volumetric space was nlin6. But that's cifti :(

[tsalo]

This is related to #1076.