Return a spectrum from SNCosmo object

[benjaminrose]

Convert to using model.flux(wave=wave, time=time)in embed2roman.py (https://github.com/benjaminrose/SN-with-Roman-Prism/commit/0f38b7a17638c866f54ca93706ff918a67bd4fc9#r43759142).

• https://sncosmo.readthedocs.io/en/stable/spectrum.html
• https://sncosmo.readthedocs.io/en/stable/api/sncosmo.Source.html#sncosmo.Source.flux

[benjaminrose]

I have "a" sncosmo.spectrum object, but waiting on #2 before pushing.

Then we need to:

• get the correct wavelengths
• convert it to a roman spec
• save spec to disk
• get many SN at many phases