Add a very simple example to edit private tags

It uses the low level interface since gdcm.Anonymizer does not allow editing of private tags
Add missing tag in RTDOSE image

As reported by: Nicky Patterson <pattersonnp@gmail.com>
GDCM would not add a Frame Increment Pointer which is required in the case of RTDOSE
Update the VTK example to show before/after the change what was needed to fix the output DICOM
Closes: #334