SlicerGeneralAnatomy segmentation context is incomplete

[fedorov]

As pointed out by @cpinter, there appear to be entries present in the current version of this file https://github.com/Slicer/Slicer/blob/master/Base/Logic/Resources/ColorFiles/GenericAnatomyColors.txt, but not present in the segmentation context: https://github.com/QIICR/dcmqi/blob/master/doc/segContexts/SegmentationCategoryTypeModifier-SlicerGeneralAnatomy.json.

Specific example: 3dSlicer label "gray_matter" is not mapped, but there are many more, most likely, since the file in the Slicer repo has 308 entries, and the segmentation context file has only 240!