Ability to load SPECT image masks

For modalities like PET, its image information spans across multiple files with a 2D image "slice" present in each file. With SPECT images on the other hand, only one file is given containing the entire 3D volume. This poses an issue when loading a 3D SPECT file mask as RT-Utils assumes that the series data is split as a file per slice.

Notes:

• We can distinguish a SPECT image via its 'NM' modality
• Instead of each slice containing its own image patient orientation, SPECT images will only have one orientation for the 3D volume. Some interpolation from this original value may be required as we iterate across each slice

@carluri Feel free to add anything you feel is missing.