ENH: Major update of Longitudinal PET/CT Analysis Extension for Slicer 4.3

DICOM Plugin updates:
- DICOMLoadable caching for performance improvement
- Selection from multiple CTs for PET
- PET only support
- Usage of PETSUVImageMaker module to generate normalized volumes

Module updates:
- Registration of PET/CT studies image volumes
- Quantification: use of PETSUVImageMaker for SUV normalized PET image volumes
- Module settings sidebar
- PET color selection presets
- CT Window/Level selection presets
- ReportOverview Table provides statistics in cells
- Additional segmentation statistics: StdDev, Count, Volcc, Volmm3
Merge pull request #438 from paulcm/update-LongitudinalPETCT-4.3

Major Longitudinal PET/CT Analysis Extension update