Towards `SpectrumBase.jl`

[icweaver]

Following up from our Steering Council meeting today, I am setting this up as a tracking issue for preparing this package to serve as a base spectroscopy package for our more specialized spectroscopy tools, e.g., PhotometricFilters.jl and SpectralFitting.jl.

Goal

Streamline functionality here to just core wavelength and flux storage, and maybe keep some small utility functionality like blackbody.

Open issues / PRs

Basically just sorting out where they should be housed moving forward):

• Unify XSpectrum types #24 - High priority
• Add resampling extension #26 - Does similar functionality already exist in SpectralFitting.jl, or should it live here?
• Tokyo Shift #28 - Should this still live here?
• Build out X-ray support (PRs incoming from Fergus)