ENH: Parse kwargs to allow no T1w

[mgxd]

This came up in nipreps/nibabies#411

Also, adds a default to the expected kwarg anat_only to allow bare initialization

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.40%. Comparing base (2ec6b23) to head (83b887e).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #922      +/-   ##
==========================================
+ Coverage   69.35%   69.40%   +0.04%     
==========================================
  Files          86       86              
  Lines        8471     8481      +10     
  Branches     1058     1058              
==========================================
+ Hits         5875     5886      +11     
  Misses       2373     2373              
+ Partials      223      222       -1     

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