update LongitudinalPETCT for 4.4

Fix issues related to VTK6 API changes

Note: Source code is now checked out from QIICR repository, as the original
developer (Paul Mercea) is unavailable to support this extension (not responsive
to emails).
Merge pull request #843 from QIICR/update-LongitudinalPETCT-4.4

update LongitudinalPETCT for 4.4