TST: Test dMRI model instantiation through factory

[jhlegarreta]

Test instantiating the dMRI models through the factory.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.97%. Comparing base (431ce51) to head (e93906b).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #252      +/-   ##
==========================================
+ Coverage   77.55%   77.97%   +0.41%     
==========================================
  Files          24       24              
  Lines        1439     1439              
  Branches      166      166              
==========================================
+ Hits         1116     1122       +6     
+ Misses        252      247       -5     
+ Partials       71       70       -1     

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[jhlegarreta]

If the PET models should also be allowed to be instantiated through the factory (related PR #250) the iterator, this will be added and modified in a separate PR as the factory will require fixes. Merging.