Full megabeast ensemble model

[karllark]

The full megabeast ensemble model needs to be implemented and documented. This includes allowing for the ensemble parameters to be input, the "simple" model implemented, and the details put in the docs.
Components of megabeast model:

• A(V) distribution (log-normal - confirm fully implemented)
• R(V) distribution (Gaussian)
• f_A distribution (Gaussian)
• star formation history (bins in log age)
• age-mass metallicity (single metallicity per log age?)

dust parameters should be coupled as our current physical model is for two sheets of dust - one in the foreground and one in the galaxy midplane. The A(V), R(V), and f_A distributions should be coupled such that the stars only seeing the foreground cloud should all be modeled by the 1st cloud A(V), R(V), and f_A distributions. The same for the stars behind the midplane cloud as they should see the combination of both clouds.