Raised Error when using RV data

[juague135]

Orbitize raises an exception when attempting to use a dataset with RV data. "Exception: If including RV data in orbit fit, set fit_secondary_mass=True"

[semaphoreP]

Note that this is the case of only planetary RV

[sblunt]

Hey Juan, finally getting around to addressing this bug! I think in the meantime, I must have fixed this somewhere along the line, because I can't reproduce it any more. I was fiddling with the companion rv implementation a lot around this time, so I'm sure it's buried somewhere in all the commit messages. I added a unit test to check this scenario and make sure it stays fixed in the future (in #363). Please reopen if you run into this issue again!