Implement specutils as an option

[simontorres]

The wavelength solution implementation is not flexible enough (at least in my opinion). So, maybe I should start talking about implementations. My idea is to keep the current way of writing the raw wavelength solution in the custom method developed for the pipeline, the iraf-compatible method and add an option to let specutils take care of the solution handling process.

It should not be the default option to not disrupt current operations but I believe at some point it should be the default.

The custom implementation is OK because the code for writing and reading are provided.