Fit Gaia astrometry of two objects together

[semaphoreP]

Is your feature request related to a problem? Please describe.
For wide separation companions, there may be Gaia positions and proper motions for both objects. We should extend orbitize to fit both sets of Gaia astrometry. Basically, similar functionality to https://github.com/logan-pearce/lofti_gaia.

Describe the solution you'd like
Somehow adapt or extend the existing Gaia module to handle two sets of Gaia measurements and not just the primary star.

Describe alternatives you've considered
We tried nothing and we're all out of ideas.

Additional context
https://github.com/logan-pearce/lofti_gaia.

[sblunt]

lol @ alternatives description. It's worth noting that we can already fit two Gaia astrometry points with just business-as-usual orbitize! functionality-- treat the primary measurement using the gaia.py module, and input the second as a relative sep/PA measurement. So we'd be adding the ability to fit the difference between the proper motions.

[sblunt]

We'd also need to add the ability to account for correlations between positions and proper motions.