Monochromatic SED

[jmeyers314]

Just one more thing I want to get in before 2.5... It'd be nice to have some syntactic sugar for creating monochromatic SEDs. As opposed to:

sed = galsim.SED(
    galsim.LookupTable(
        [0, 621, 622, 623, np.inf],
        # [0, 621, 622, 623, 10000],
        [0, 0, 1, 0, 0],
        interpolant='linear'
    ),
    wave_type='nm',
    flux_type='fphotons'
)

Now this could simply be the above, or it could be a whole galaxy emission line interface with Gaussian/Voigt/Lorentzian/... 1-d profiles. Probably above is sufficient, but wanted to discuss briefly here before committing anything.

[rmjarvis]

I'd suggest a subclass. galsim.EmissionLine maybe. For now we could just have it do the above given a center wavelength, equivalent width, and maybe something to say how sharp it is. (E.g. FWHM, which could default to 1 nm as above.) If the sawtooth shape isn't good enough for some purposes, we could let people extend this later to have other line shapes, but probably no strong impetus to do anything complicated yet.

[rmjarvis]

Done. #1249