ENH: Add fsaverage to default output spaces

[mnoergaard]

This PR addresses issue #255 by including fsaverage to the list of default output spaces. With the (hopefully) increasing use of PETSurfer-BIDS (https://github.com/freesurfer/petsurfer-bids) it would be ideal to have these outputs readily available for analysis.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.06%. Comparing base (7efcfc0) to head (2ca1f75).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #258   +/-   ##
=======================================
  Coverage   83.06%   83.06%           
=======================================
  Files          87       87           
  Lines        8177     8177           
  Branches      873      873           
=======================================
  Hits         6792     6792           
  Misses       1163     1163           
  Partials      222      222           

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