ENH: Add support for other petref images for registration

[mnoergaard]

This PR addresses issue #184 and #177 by adding support for 1) using other reference PET images for e.g. co-registration, and 2) implementing an additional option for performing PET to anatomical co-registration using ANTs. It implements the possibility to use 'twa' which is a time-weighted average of the motion corrected PET data. Furthermore, it also allows the use of 'sum' which is a summed image across time for the motion corrected PET data. The default is still the template generated from the motion correction using mri_robust_template.

[codecov]

Codecov Report

❌ Patch coverage is 87.15084% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.78%. Comparing base (133526d) to head (57a40cf).

Files with missing lines	Patch %	Lines
petprep/workflows/pet/fit.py	82.43%	6 Missing and 7 partials ⚠️
petprep/interfaces/reports.py	60.00%	3 Missing and 3 partials ⚠️
petprep/cli/parser.py	33.33%	1 Missing and 1 partial ⚠️
petprep/workflows/pet/registration.py	94.44%	1 Missing ⚠️
petprep/workflows/pet/tests/test_fit.py	98.43%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #185      +/-   ##
==========================================
+ Coverage   80.63%   80.78%   +0.14%     
==========================================
  Files          84       84              
  Lines        6512     6672     +160     
  Branches      657      678      +21     
==========================================
+ Hits         5251     5390     +139     
- Misses       1097     1107      +10     
- Partials      164      175      +11     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.