Add support for DICOM SEG mask files.

[alexzwanenburg]

Masks in DICOM are either stored as Radiotherapy Structure (RTSTRUCT) objects, or as Segmentation (SEG) objects. mirp currently supports the RTSTRUCT DICOM files as masks, but not SEG files.

The following is required:

• Find a (public) dataset that contains an image with its corresponding SEG object.
• Write a unit test to start implementation (in test/import_mask_test).
• Fully implement the mirp.importData.imageDicomFileSEG.MaskDicomFileSEG class and its class-specific methods.
• Add additional unit tests for testing import of SEG files in various circumstances.
  • test/extract_mask_labels_test.py
  • test/import_image_and_mask_test.py
  • test/read_image_and_mask_test.py
• Update long-form documentation to mention the use of SEG files.

[alexzwanenburg]

SEG files can be read using MIRP.