Replace our custom calc_moon with new moon98 in SOFA 18

The latest SOFA has a new moon98 routine that calculates position and velocity using the Meeus algorithm. This can replace our own Meeus implementation in coordinates.orbital_elements - should give the same (or very similar) results and has the benefit that we'll also get velocities.

Milestoning 5.0 since pyerfa will definitely be up to that state by then (ERFA update is waiting for review; liberfa/erfa#84).