Handle ra/dec calls gracefully in anticipation of NonFixedTargets

[bmorris3]

(You can assign this issue to me!)

Make a function that unpacks coordinates from FixedTargets (like here, for example) that can take a time as an argument, so that NonFixedTargets can use the same machinery as FixedTargets.

[eteq]

For anyone else looking at this, the context is a conversation @bmorris3 and I had - the idea is that this will let us press forward on the way we just discussed in the hangouts without really dealing with the NonFixedTarget issue, but make it much easier to address it later because it will require only changing that one place.