Create an automatic sherpa model generator

[zblz]

From the radiative and functional models, create a sherpa model automagically, e.g.:

>>> from naima.models import create_sherpa_model, Synchrotron, PowerLaw
>>> SynPL = create_sherpa_model(model=Synchrotron, particle_distribution=PowerLaw)

All parameter fields, etc, should be populated programatically from the given models.

[cdeil]

@zblz – now that Sherpa is open, we'll start using it heavily in Gammapy.

@kingj90 will work on gammapy.spectrum ... we should try to find a way to avoid duplicating spectral model classes and functionality in gammapy and naima.

[zblz]

@cdeil - definitely, we should not duplicate this. I did a few enhancements on the sherpa models in naima so that they are easier to code (base classes for each particle distribution and then a subclass for emission process) but this is not meant to be exposed to the user. I was not planning on developing this further than the thin wrappers in place in the near future, so if you have a plan for a better sherpa model framework we could use that with the naima models as well.

[cdeil]

@joleroi, @adonath and I were discussing this briefly today.
I think a factory function or some other Sherpa model generator would be nice.
If it's more that 20 lines of code / significant effort, one should first look at https://saba.readthedocs.io/
That was a GSoC project and had input from both Astropy and Sherpa devs.
I haven't looked in detail what they do or if it's suitable for our needs.