Nicer api

[simonrw]

Add a nicer api

This returns a namedtuple of the results, with the following contents:

planets
epochs
times
rsky
vsky
rv (may be None)

Each one is a numpy array for speed, but this does add the numpy to the requirements. This is probably ok as people who want this library are probably already using numpy.

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This change is 

[simonrw]

Rebasing from master