Add Python classes for XSpec convolution models

[DougBurke]

The XSpec models come in additive, multiplicative, convolution, pileup, and mixing-model types. Sherpa already supports additive and multiplicative. It does not support the one pileup model provided by XSpec, but does have its own version of this functionality. This leaves convolution and mixing-model types. This request is about the convolution models only, which are documented at http://heasarc.gsfc.nasa.gov/docs/xanadu/xspec/manual/Convolution.html

The _xspec.cc file already contains interfaces to the XSpec convolution models. To support there use in Sherpa we need Python classes.

I have an initial set of classes in my branch at https://github.com/DougBurke/sherpa/tree/feature/xspec-convolution-models - which is based on the support I wrote for these models in the CIAO contributed-script package. It is not ready for a PR, as I want to experiment a bit more.

Note that to properly use these models from Sherpa will require a rethink of how model evaluation is done (I will write up thoughts on this in another issue at some point in the near future). That is, some XSpec convolution models can be used as is, but some aren't going to be useful until further changes are made in Sherpa itself.

The gist at https://gist.github.com/DougBurke/65dd0716afc061a03d63 contains the output of a script (and the script itself) that shows the cflux and cpflux models being used - see http://heasarc.gsfc.nasa.gov/docs/xanadu/xspec/manual/XSmodelCflux.html and http://heasarc.gsfc.nasa.gov/docs/xanadu/xspec/manual/XSmodelCpflux.html using DougBurke@ccd2ccf

[DougBurke]

I've updated the gist at https://gist.github.com/DougBurke/65dd0716afc061a03d63 to add example2.py, which is a version of the script using the API added in changeset DougBurke@810cae9

[DougBurke]

I wonder if there's overlap with #43 - at least in the sense that the output grid may not be the same as the grid we want to evaluate the data on.