ENH: Add robust co-registration between PET and MRI

[mnoergaard]

This PR addresses issue #177. Sometimes the co-registration between PET and anatomical T1w might fail due to low signal or large initial misalignments. In these cases, it would be ideal to have an option to perform robust co-registration.

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.40%. Comparing base (f895f87) to head (9e7561f).

Files with missing lines	Patch %	Lines
petprep/cli/parser.py	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #178      +/-   ##
==========================================
+ Coverage   80.23%   80.40%   +0.16%     
==========================================
  Files          84       84              
  Lines        6366     6398      +32     
  Branches      638      643       +5     
==========================================
+ Hits         5108     5144      +36     
+ Misses       1096     1092       -4     
  Partials      162      162              

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