Add PETDICOMExtension

The PET DICOM Extension provides tools to import PET Standardized Uptake Value images from DICOM into Slicer:
*SUV Factor Calculator CLI - takes a PET DICOM series and creates a Real World Value Mapping (RWVM) file including 4 types of Standardized Uptake Value conversion factors (SUVbw, SUVlbm, SUVbsa, SUVibw).
*DICOM RWVM Plugin - extends the DICOM module to support parsing and loading of DICOM series associated with Real World Value Mapping (RWVM) objects.
*DICOM PET SUV Plugin - extends the DICOM module to support parsing and loading of PET DICOM series as Standardized Uptake Value (SUV) images.