Sourcetools density methods don't allow setting metallicity in cluster_cr_conv calls

[DavidT3]

Essentially this means that for all of the density convenience functions, a metallicity of 0.3 solar is assumed.

That is a perfectly reasonable value to assume for clusters, but when you get down to the low temperature group regime the choice of metallicity has an outsize effect on the impact of radiative cooling, and thus on the conversion factor calculated to take the emissivity to the density.

Clearly the metallicity has to be configurable in those density convenience functions.