ENH: Allow alternative extensions when creating a BIDS skeleton

[mgxd]

This allows creating non-nii.gz files through generate_bids_skeleton. If no extension is provided, .nii.gz will still be used as the default.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.93%. Comparing base (bbe6170) to head (1958365).

❗ Current head 1958365 differs from pull request most recent head e62aa9b. Consider uploading reports for the commit e62aa9b to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #858      +/-   ##
==========================================
- Coverage   60.96%   60.93%   -0.04%     
==========================================
  Files          55       55              
  Lines        6558     6558              
  Branches      954      954              
==========================================
- Hits         3998     3996       -2     
- Misses       2374     2380       +6     
+ Partials      186      182       -4     

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