Make time consistent in floating-point seconds since epoch across adapters and datasets

[milancurcic]

See #282 (comment)

We should consistently across datasets return time in xr.Datasets as floating point seconds since epoch (rather than decoding to np.datetime64[ns].

Users can still decode to np.datetime64[ns] if they want and the functions that accept time (and support the same set of arithmetic operators) will continue to work.