WIP: Add gray matter threshold option to reference mask workflow

[mnoergaard]

This PR adds support to select a gray matter threshold for the generation of a reference mask (reference mask workflow). The extension is useful when a given segmentation (e.g. thalamus in FreeSurfer) does not follow gray matter boundaries, and so additional modifications to the region definition may be necessary.

[codecov]

Codecov Report

Attention: Patch coverage is 95.91837% with 2 lines in your changes missing coverage. Please review.

Project coverage is 77.62%. Comparing base (643e14b) to head (b22e544).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
petprep/utils/reference_mask.py	71.42%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #50      +/-   ##
==========================================
+ Coverage   77.48%   77.62%   +0.14%     
==========================================
  Files          80       80              
  Lines        5849     5895      +46     
  Branches      612      615       +3     
==========================================
+ Hits         4532     4576      +44     
- Misses       1172     1173       +1     
- Partials      145      146       +1     

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