Swap out planets_and_moons.txt for planets_and_moons.yaml

[ktfhale]

As mentioned in Ticket #156, in which planets_and_moons.yaml was created

Eventually, we should transition to using only planets_and_moons.yaml. At the moment, planets_and_moons.txt is still used at least by get_planets_and_moons_list(), which is used at least by get_mtargs(). This should go in a separate ticket.

This should not be particularly difficult. We should probably grep the entire pipeline to make sure it's not being used anywhere we don't expect.