ENH: make the generation of fsaverage outputs default behaviour

[mnoergaard]

What would you like to see added in this software?

With PETSurfer-BIDS coming along (https://github.com/freesurfer/petsurfer-bids), we might consider generating fsaverage outputs by default, i.e. add fsaverage to standard output spaces. This would allow for more direct use of https://github.com/freesurfer/petsurfer-bids, instead of users having to recompute this step, despite being fast with the precomputed transforms.

Pros

1. Will allow for direct surface-based modeling using https://github.com/freesurfer/petsurfer-bids

Cons

1. Will slightly increase computational time (but only by very little, because the transforms to fsaverage have already been computed)
2. Will increase the number of file outputs and thereby storage. However, often the surface files take up very little storage.

Thoughts @mathesong , @effigies , @CPernet ?

Do you have any interest in helping implement the feature?

Yes

Additional information / screenshots

No response

[CPernet]

So very little extra time, little extra space, seems like get go to me.

Seems to complement well the parcel based --> petfit and surface --> petsurfer