Deprecate and remove NiftyPET functionality

[KrisThielemans]

NiftyPET/NIPET has undergone major design changes, with less code in CUDA and more in Python. Unfortunately, that means that we can no longer call relevant routines from C++ (without going through major hoops, anyway).

As we now have GPU support via parallelproj, the effort required to update the STIR wrapper for NIPET seems too high.

More info on NiftyPET/NIPET#52