Consider unit handling for spectra

[dr-rodriguez]

Units for spectra are, for the single Spex Prism example, extracted from the FITS header. This should be an optional parameter that can be passed to set it. Ideally, as either a column name or an actual string value.
Error handling should also be in place in case the unit can't be resolved by astropy.

[dr-rodriguez]

The latest commits ensured that error handling was in place to avoid missing/invalid units. It's not clear how to handle passing along your own units, especially since long-term the spectrum handling code would be through specutils. So I will resolve this as a wont-do and we can revisit it later if the need arises.