More flexible units, less hardcoding

[gully]

We currently assume Angstroms everywhere in gollum. While that is convenient, some communities are used to microns or nanometers, or maybe even wavenumbers. We should allow the spectra to reside in whatever units are input. We'll have to sanitize any locations that have Angstroms hardcoded, that's doable. Wavenumbers are harder to deal with because in some places we may assume spectral axes are sorted in certain ways.

The flux units are also tricky. We should consider allowing either $F_\nu$ or $F_\lambda$ values.

These are all straightforward to implement with the astropy equivalencies protocol, it may just require some boiler plate code whenever we do some operation that may depend on the unit (e.g. plotting, black body normalization).