PixelData should not be precached

[pieper]

The dicom module uses the plugin's tags member to know which fields to put into the TagCache database:

https://github.com/Slicer/Slicer/blob/bc6482c9a4abaabe3f93b0bfee31d5a8a2c4ae97/Modules/Scripted/DICOM/DICOM.py#L176-L185

But the PETSUV plugin includes PixelData in the tags member:

https://github.com/QIICR/Slicer-PETDICOMExtension/blob/master/DICOMPETSUVPlugin/DICOMPETSUVPlugin.py#L65

This probably leads to a large tag cache and poor performance, although I haven't timed it.

Possibly related to #21