Fix bug in Model.submodel_names when the compound model has multiple copies of the same model
Added an isinstancemethod function to astropy.utils.introspection that I need for use in the modeling package
Initial work at fully supporting parameters on composite models.  The prescription for generating new parameter names from the submodels' parameters is very primitive for now and could and should be improved.  But more importantly it *works*.
Preserve model linearity for compound models, for the most part
The minimal bits needed to allow compound models to be fitted.
Just some (mostly) trivial style cleanup in the tests, with one test outright removed since it was a trivial test of the tests.