Provide recycling fractions to get stellar masses

[christopherlovell]

Parametric histories in particular are defined in SFR space, so to get the current stellar mass requires an assumed recycling fraction. This will be SPS model dependent.

We could provide a few common recycling fractinos (with metallicity dependencies) to apply to your given SFH in order to get the current stellar mass.

This is of particular importance when using synthesizer for inference of stellar masses. Thanks to Kartheik Iyer for pointing this out.