Extension / new package to compute BCs from synthetic spectra

[cgarling]

In BEAST, which fits stellar parameters for single stars, theoretical isochrones are used in combination with libraries of synthetic spectra (see here, here) and a defined set of filters (see here) to cast the models into the observational space. By going down to the level of the spectral models, they can apply whatever reddening law they want rather than being constrained by whatever is baked into pre-computed BCs (e.g., the MIST BCs). This would be nice to support but I think is beyond the scope of this single package.

The spectral library component could potentially fit under Spectra.jl since its description includes spectral libraries but I don't currently see any related functionality. It would be nice to maintain the filter sets and related functionality in their own package I think.

[cgarling]

Photometric filters can be provided through PhotometricFilters.jl.

[cgarling]

PhotometricFilters.jl uses the pyphot/pyphot/libs/new_filters.hd5 file from pyphot, which hasn't been updated in 4 years. pyphot also has an interface to the SVO filter service which would be a nice way to acquire the filters. SVO allows HTML queries to obtain the filters in VOTable format (see here). In any case, we want to make sure we have a way to obtain the most up-to-date filter curves available (e.g., JWST post-launch filters rather than pre-launch), ideally without having to manually check the filters into the repo.

We also want to make sure we use the correct reference spectrum for Vega, see here, section 9.1.1 here, and info on JWST zeropoints here. The current CALSPEC files are hosted here and I think this is the most recent Vega spectrum.