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

The model factory is only designed to instantiate dMRI models.

Author: jhlegarreta

src/nifreeze/model/base.py

@@ -67,7 +67,7 @@ def init(model: str | None = None, **kwargs):
 
             return AverageDWIModel(kwargs.pop("dataset"), **kwargs)
 
-        if model.lower() in ("gqi", "dti", "dki", "pet"):
+        if model.lower() in ("gqi", "dti", "dki"):
             from importlib import import_module
 
             dmrimod = import_module("nifreeze.model.dmri")