Helper function to work with tau

[semaphoreP]

Tau is great for sampling, but somethings annoying to work with. Instead of having each person re-figure out tau every time, it'd be great to have some helper functions in orbitize. I was thinking of the following

• Convert tau to T0 (actual MJD date of periastron passage). Optionally, T0 can be the first periastron passage after a certain date.
• Convert tau to a different tau (we base tau off of MJD=0, but having the flexibility of switching to a different date is also good).

I think the first one is the most important, but the second one could also be useful while we are attacking this problem.

[semaphoreP]

Fixed in PR #105