Absolute magnitude from generic isochrone color

[kadrlica]

@sidneymau ran into the problem that the absolute magnitude calculation requires that the isochrone uses g and r band magnitudes assuming the DES system. We can work around either by having transformation equations for other bands, or by accessing the g and r magnitudes directly from the isochrone. The use of sample makes this a bit more complicated, so maybe we need to make a copy of the isochrone with band_1 = g and band_2 = r.

[kadrlica]

Also see #16