tighter astropy integration and better time slicing support

[telegraphic]

Astropy has ndarray-based classes for time, coordinates, and quantities. We could use these and return things like frequency axis data with units attached.

An extension of this is to allow plotting axes with desired units and formats (e.g. Hz, MHz, MJD or unix time).

We should also allow time slices by index (t_start=0, t_stop=16384) or by other formats (t_start=57767.1234, t_format='MJD') or something like that.