Implementation of Hypatia PDF

The Hypatia PDF is generalization of the crystal ball PDF and it is used for modelling mass resonances like the Jpsi and the B0 mass and is used by multiple LHCb working groups: https://arxiv.org/pdf/1312.5000.pdf

Most often this implementation is used which contains a lot of commented-out code:

https://gitlab.cern.ch/lhcb/Urania/blob/master/PhysFit/P2VV/P2VV/RooIpatia2.h

https://gitlab.cern.ch/lhcb/Urania/blob/master/PhysFit/P2VV/src/RooIpatia2.cxx

This is the double tailed version, in the paper only the single-tailed version is really mentioned, but both can be useful. Since this is such a crucial function, it would be great to have a standardized version in RooFit.