Supporting XSPEC user models

[DougBurke]

So, I recently split up the module to add xspec-models-cxc-helpers in part to help with building modules that provide access to XSPEC user models. This is based on CIAO where Sherpa provides the code needed to build a module - at least the generic parts - and there's a separate package for actually building such a module. It's not yet clear to me whether this will actually work - I think I got disillusioned but I can't remember why. I do know that this sort of code doesn't really fit into the current thinking around python packaging (since such a module doesn't really make sense for a wheel since it depends on so many moving parts), but maybe that's just my own biases and neuroses showing!