specwcs update to new modeling API

This is updating specwcs to the new modeling API. As discussed with @embray there are still problems with `Quantity` that need to be addressed, but otherwise I think `Model` is getting there. 

I will use this PR as a base for a restructure in Spectrum1d (using `.wavelength`, `.frequency`, `.energy` as discussed with @keflavich).

there are some questions in my specwcs.py comments - please comment if you have an answer.