REF: generalised anat_fit_wf

[eilidhmacnicol]

This is a first-step towards having anatomical workflows be less biased towards t1w in preparation for extending to populations other than adult humans

It adds a preferred anatomical image type reference_anat to the fit workflow, and then changes the calls to t1w or t2w as either the reference anat or the auxiliary anat, if there is more than one type of anatomical image present

Edited to add:
the goal here is trying not to break the existing workflow. we'll add more functionality later

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.32%. Comparing base (5292c7b) to head (3d8ca2d).

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #475       +/-   ##
===========================================
+ Coverage   63.21%   74.32%   +11.10%     
===========================================
  Files          25       25               
  Lines        1952     1963       +11     
  Branches      244      247        +3     
===========================================
+ Hits         1234     1459      +225     
+ Misses        679      436      -243     
- Partials       39       68       +29     

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