clean separation between sasmodels and sasview

[pkienzle]

sasmodels should act like an indpendent third party package without knowledge of sasview internals. This is not the case in a few places:

• sasview_model.py belongs in sasview. It was developed within sasmodels for convenience, but now that sasmodels is mature it can be moved to sasview.

• sasmodels should not have knowledge of the sasview data representation. Currently this infects resolution.py, resolution2d.py and sesans.py. data.py was set up as a clone of the sasview data structures for testing, and this has infected direct_model.py, bumps_model.py and compare.py. Perhaps we could create a sasdata package which both sasview and sasmodels depend upon, or perhaps the data representation could move (along with the loaders?) to sasmodels.

• compare.py imports models from sasview 3.x for test purposes; this code will eventually disappear once we are certain we no longer need to compare against 3.x

• convert.py includes knowledge of the 3.x model parameters and how they relate to the new sasmodels parameters. This information should stay in sasmodels, along with any future model migration code.

• sasmodels documentation currently living in sasview/src/sasgui/perspectives/fitting/media/plugin.rst mostly belongs in sasmodels docs.