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 80.63%. Comparing base (153cd71) to head (d9393f3).
⚠️ Report is 9 commits behind head on main.

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   80.57%   80.63%   +0.06%     
==========================================
  Files          84       84              
  Lines        6480     6512      +32     
  Branches      652      657       +5     
==========================================
+ Hits         5221     5251      +30     
- Misses       1096     1097       +1     
- Partials      163      164       +1     

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