Spacetime generic disc velocities

[fjebaker]

At the moment, the disc velocities for a disc in the equatorial plane are assumed to have circular photon orbits in the Kerr metric. This should be made generic for any arbitrary metric, so that redshifts for thin accretion discs may be calculated beyond Kerr.

[fjebaker]

#29 sort of does that with automated orbit discovery, but only works outside of the plunging region for now.

Also have some generic mathematical models that can be used here, but those are a bit of work away from being refined.