Add derived parameters to the results dictionary

[jvines]

I think it would be a good idea to add the derived parameters (e.g. p and b when using r1 and r2) to the results.posteriors dictionary, which can be a time save for the user if they want to produce a cornerplot or individual distributions.

[nespinoza]

True. Will mark it as a to-do enhancement!

[jkteske]

Second this! It would also be useful to include, say, Mp if Mstar is provided.

[jvines]

Ah, yes! That's something I've thought of as well. I guess I got too accustomed to doing the calculation myself. It would be a very very welcome addition!