FIX: Avoid trying to instantiate the PET model through the factory

[jhlegarreta]

The model factory is only designed to instantiate dMRI models.

[jhlegarreta]

Unless these are vestiges from a pre-PET design:
https://github.com/jhlegarreta/nifreeze/blob/1ca869466159d706264f843dacf3c6feeac595b5/src/nifreeze/model/base.py#L42
https://github.com/jhlegarreta/nifreeze/blob/1ca869466159d706264f843dacf3c6feeac595b5/src/nifreeze/model/base.py#L73

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.65%. Comparing base (a2bae01) to head (0eec34a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #250   +/-   ##
=======================================
  Coverage   78.65%   78.65%           
=======================================
  Files          24       24           
  Lines        1443     1443           
  Branches      166      166           
=======================================
  Hits         1135     1135           
  Misses        237      237           
  Partials       71       71           

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