Add background density to results

[kadrlica]

It would be nice to add the background density to the results file. This is a bit complicated, because we are actually calculating the background model as a 2D probability distribution in color magnitude space, but maybe we could do something like sum loglike.b values for stars in the interior region?

This feature was requested by @scantu and @sidneymau in order to better plot radial profiles including the background density.