coordinate transform

[benjaminpope]

Trivial issue here - I want to add to_angular and to_cartesian methods to BinaryModel* to convert between them. Seeking feedback on whether we want this method to return an object, a dict (ie {'dra':...,} or {'sep':...}) or just an array [dra, ddec, flux]. Mainly want this to pass trivially from a grid search in cartesian to MCMC in angular coordinates.

Would probably also be a way easier way to do the azimuthal averaging.