Explore moving out units and/or plotting

[icweaver]

• Put 'em in extensions
• Switch to Makie plots

[cgarling]

I would wait to deal with units until we have a better idea of what other operations (smoothing, integration, line fitting, equivalent widths, etc) we want to support. Some operations on spectra require knowledge of whether the provided spectral data are in flux density or energy density units, see for example here and the calculation of synthetic magnitudes from spectra here.

If we want to interface Spectra.jl with PhotometricFilters.jl, it may be necessary to always have units associated with spectra. We may be able to support multiple unit packages through extensions, but it will be clearer how to implement the units once more of the features are developed.

[icweaver]

Thanks for the examples! Yea, agreed, it'll be good to see how things shake out first. I marked this as Post-Registration, but maybe I should name it Way-Post-Registration