More array support?

[icweaver]

Following up from #24, just saving this here in case doing something like subtyping AbstractArray and/or updating the 2 and 3-arg show functions a bit would be helpful here

Update: Also wrapping #3 (comment) into this to explore at some point

[cgarling]

We decided not to subtype AbstractArray in PhotometricFilters.jl (see here) which was trying to solve a similar problem (a filter is defined by 1-D wavelength and throughput vectors; a 1-D spectrum is defined by 1-D wavelength and flux vectors).

If we want to support general multi-dimensional spectra maybe subtyping abstractarray would be good but otherwise I probably wouldn't

[icweaver]

Thanks for the example! Yea, I was sort of leaning towards not subtyping it too for similar reasons, but will keep this around just in case we try merging Echelle and Spectrum post-registration and want to explore if it has any benefits