convert LinearWCS to Polynomial model

In this PR I have ditched my self-programmed `Spectrum1DLinearWCS` in favour of using a @nden's `Polynomial1D` based on `Spectrum1DPolynomialWCS`. The former has its `__init__` function deprecated (@embray a class based deprecator would be awesome) to ensure that this will be phased out. 

I have also now used this new wcs in the read_fits functions. Finally, I have marked several tests for skipping as they need to be updated to fit within the new paradigm's.

@mhvk @keflavich - if you could have a quick look. 

I will clean up some other smaller things before dealing with documentation and tests (separate PRs) - before presenting this.