Dynamical mass framework

[semaphoreP]

This just modifies the system class to allow for fitting a companion mass with future improvements to the API. It does a few simple things:

• keyword to tell system class to add in a variable to fit for mass
• a optional prior for the mass of the second object
• expanded the docstring to indicate where the mass priors should go in the list of priors.

Basically, we can access orbital elements from the front, and access masses using -1, -2, -3....

Other than that, it does nothing else. We need to implement the hooks to fit stellar RV and astrometry to actually constrain the mass.