Move Earth computation into Planet.__call__()
Move Moon computation to Planet.__call__()

Also, remove the duplicate computations of earth_share and moon_share
that were attached to the Skyfield ephemeris object and that,
regrettably, swapped the two formulae so that our previous formulae
worked but said the opposite of what they ought.
Add Planet.observe_from() that wraps observe()
Abandon observe() and move code to observe_from()