jplhorizons epochs kwarg should accept Time objects

`'epochs'` should be `Time` objects or at least be able to accept `Time` objects.

Some discussion was here: #1488

Start with the discussion in #1488, then locate the JPL Horizons query entry point that handles the `epochs` kwarg. Establish the expected behavior for `Time` objects from that discussion; done means the kwarg accepts them as requested and the relevant behavior is covered by tests.

Written by the indexing model from the issue text.