NRRD and STL files open as segmentation by default

[agirault]

Issues only happens in NIRFASTSlicer, not in 3DSlicer.

Any nrrd file will be loaded as a segmentation by default instead of a volume, while a stl file will be loaded as a segmentation by default instead of a model:

Loading the stl as a segmentation will work (even though we'd prefer defaulting to model), while loading the nrrd as a segmentation will fail:

[agirault]

• Ensure segmentation is not default
• Fix crash when loading as segmentation Slicer/Slicer@28041c8#r28453881