ENH: init quantity/units in scalar volume plugin
Created by: fedorov
Scalar volumes loaded from various various flavors of MR and CT modalities will be asigned DICOM.QuantityCode and DICOM.UnitsCode to describe the pixel values stored in the volume. A function mapping SOPClassUID is introduced in the base DICOM plugin class to perform the mapping.
See related discussion in
https://github.com/QIICR/QuantitativeReporting/issues/127#issuecomment-273925207
A similar approach is implemented in the PET and Parametric Map plugins provided in the extensions.