Last call for name changes in modeling

This addresses #1652 and #1677.

I have removed `Model` from the names of all models except base classes and composite models for which I think it's clearer to keep it and they are not often used directly. `ParallelCompositeModel` was renamed to `SummedCompositeModel` and it now returns only the sum of the deltas.

I am open to suggestions for other name changes but I think it would be best to do all these before 0.3.