Add support for SPECT images.

[alexzwanenburg]

mirp supports various modalities. Though positron emission tomography (PET) is currently supported, single photon emission computed tomography (SPECT) is not. DICOM removed support for SPECT directly, and these images are now part of a more generic nuclear medicine (NM) files.

The following is required:

• Find a (public) dataset that contains NM files (corresponding to SPECT).
• Write unit tests to start implementation (in test/dicom_nm_test.py)
• Implement a new mirp.importData.imageDICOMFileRTDOSE.ImageDicomFileNM class that inherits from mirp.importData.imageDicomFile.ImageDicomFile.
• Implement new mirp.images.nmImage.py contents.
• Add additional unit test for testing import, reading and processing of NM files.
  • test/extract_image_parameters_test.py
  • test/import_image_and_mask_test.py
  • test/import_image_test.py
  • test/read_image_and_mask_test.py
• Update long-form documentation to mention the support for NM images.

[alexzwanenburg]

This issue will be worked on in the long-term -- in the current DICOM standard (v3.4) there is no consistent reference for 3D reconstructed SPECT imaging.