Enable generate_dummy_segmentation_extractor to generate voxel masks

[h-mayorquin]

We should add the capability for the dummy segmentation extractor to generate voxel masks. Problem right now is that SegmentationExtractor is based on image masks as get_roi_pixel_masks assumes that _image_masks exists already:

roiextractors/src/roiextractors/segmentationextractor.py

Line 148 in 0d50073

return _pixel_mask_extractor(self.get_roi_image_masks(roi_ids=roi_ids), roi_ids)

We should probably modify this so get_roi_pixel_masks first tries to call _pixel_masks before doing trying to get them from _image_masks.. This is important in its own right as it will allow us to have segmentation extractors that can return pixel masks without us requiring to define image_masks as a workaround. Moreover, we will be able to add a flag to generate_dummy_segmentation_extractor and just build a pixel_masks with voxel_mask or pixel_mask qualities over there.