Support for transit duration variations

A feature request from @gjgilbert.

A more general interface could be to allow **any** of the orbital elements to be a function of transit number. Right now, transit time is hard coded as the only one to vary, but it shouldn't be too bad to add impact parameter (or inclination? or both? things start to get a little tricky) as another parameter that varies. I'm inclined to think that the more general interface might be easier to maintain in the long run, but I'm open to suggestions!

The issue names no files, tests, or entry points. Start by tracing how transit time and orbital elements are represented and varied by transit number; done would require an agreed interface for varying additional elements, with its behavior and scope settled before implementation.

Written by the indexing model from the issue text.