Allow for arbitrary orbits in initialization

We currently allow two types of orbits:

1. Passing a path to an orbit file as a string. The PPRs are read from the orbit file.
2. Passing None, for static arms. The PPRs are taken as constant for a fit of Keplerian orbits around t = 0.

We should remove the last option, and replace it with arbitrary orbits, relying on ForEachMOSA (a scalar, a dictionary, an HDF5 dataset, etc).