Handle ra/dec calls gracefully in anticipation of NonFixedTargets

(You can assign this issue to me!)

Make a function that unpacks coordinates from `FixedTarget`s (like [here](https://github.com/bmorris3/astroplan/blob/vectorize/astroplan/core.py#L268), for example) that can take a time as an argument, so that `NonFixedTarget`s can use the same machinery as `FixedTarget`s.

Start with the coordinate-unpacking code in astroplan/core.py around line 268 and trace how FixedTarget calls are handled. Determine the interface needed for a time argument and how it should support the anticipated NonFixedTarget path; done means the shared machinery can handle both target types without special-case failures.

Written by the indexing model from the issue text.