Add skull_file output to fsl.BET interface

[stilley2]

Summary

Adds a missing output to fsl.BET.

The skull parameter tells BET to output a skull image. However, there was no corresponding output parameter for that image. This PR adds it.

Acknowledgment

• (Mandatory) I acknowledge that this contribution will be available under the Apache 2 license.

[effigies]

Can you make specs?

[codecov]

Codecov Report

Merging #3095 into master will decrease coverage by <.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #3095      +/-   ##
==========================================
- Coverage   67.86%   67.86%   -0.01%     
==========================================
  Files         295      295              
  Lines       39260    39263       +3     
  Branches     5171     5172       +1     
==========================================
+ Hits        26644    26646       +2     
- Misses      11911    11912       +1     
  Partials      705      705

Flag	Coverage Δ
#smoketests	51.23% <66.66%> (ø)	⬆️
#unittests	65.09% <33.33%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
nipype/interfaces/fsl/preprocess.py	81.65% <66.66%> (-0.07%)	⬇️

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[effigies]

Hi @stilley2, sorry to do this to you, but we just merged some pretty huge changes. Could you re-apply these changes on the latest master? You'll want to re-run make specs entirely, and we're now using the Black formatter. You can run it manually, or use pre-commit to have it automatically run every time you commit.

Please let me know if you need any help or clarifications.