add contrast to mass limits functionality

[wbalmer]

a la plot_contrasts_mjup from pynrc.nb_funcs with various models (what does Carter et al. use for sub Jupiter mass predictions? is that Linder et al. or something else?).

[kammerje]

I think they are using the BEX evolutionary models. I can send them to you!

[wbalmer]

Discussion a while back indicated this would be a bad idea, since these transformations are model dependent and the models are discrepant from one another, we'd be making some implicit judgement call that would likely impact unwitting publications.