Store the calculated abundance factor when using variable radiative losses

[jwreep]

An oversight on my part when we first programmed this, but we should definitely store the abundance factor with the other basic variables so that it can be more easily plotted (rather than recalculating in post-processing).

[wtbarnes]

Agreed. This can be optionally returned if the variable abundance option is specified in the same way that the DEM results are only returned if that option is turned on.