Request: Add Coordinate Conversions

[jenniferyee]

It would be helpful to have utilities to convert center of magnification <--> center of mass coordinates and also to primary as the origin. Maybe also to correct the alpha problem...

[rpoleski]

Alpha +- 180 deg. issue (i.e., fully switching to Skowron+11 conventsion) can be corrected when we go from version N.A.B to N+1.0.0 and the other changes will be significant enough that most old user codes won't work.

[rpoleski]

@jenniferyee - I've coded a version of example 16 in which you can easily use user-defined parameterization (e.g., t_0, u_0 relative to planet): link. Example 16 allows fitting models using EMCEE or pyMultiNest. If you want to use that, then let me know and I'll finish this branch, merge it, and add tutorial on how to use it for different cases. This is of course far from optimal solution.