Change DegreesOfFreedom to take Points instead of Vectors

The name `EarthMoonBarycentricFrame` no longer makes much sense, since the choice of an inertial reference frame is abstracted away when using `Point`. That template parameter refers to a vector space (a choice of axis directions), see for instance the definition of such a vector space at https://github.com/mockingbirdnest/Principia/blob/master/testing_utilities/solar_system.hpp#L11-L18.

In this case, the y axis is from the Earth to the Moon at the inital epoch, the x axis is in the direction of the initial velocity of the Moon relative to the Earth, and the z axis is determined by whichever handedness you prefer.
How about `EarthMoonOrbitPlane` or something along those lines?