initialize SegmentLabel from SegmentDescription

Without SegmentLabel in JSON, dcmqi currently uses the heuristic to populate
it from SegmentedPropertyTypeCodeSequence>CodeMeaning, which is suboptimal, since
it does not take into account laterality. At the same time, SegmentLabel is used
both by OHIF and Slicer (see https://github.com/QIICR/QuantitativeReporting/blob/master/DICOMPlugins/DICOMSegmentationPlugin.py#L202)
to initialize segment names shown to the user. Related to this PR that will
revisit the heuristic used by dcmqi, and use SegmentDescription if SegmentLabel
is not available before defaulting to segmented property type code meaning:
QIICR/dcmqi#488