Simplified FourVelocity expressions for Circular Schwarzschild
KerrGeoOnSeparatrixQ: Private function to stop KerrGeoOrbit from evaluating on the separatrix (except for Circular Schwarzschild where everything is well behaved)
Made FourVelocity.m use the functions from OrbitalFrequencies.m to get rid of repeated code
Made FourVelocity API more user friendly by removing "Covariant" option and outputting all components, both contravariant and covariant, as a single association.
Partially fixes Issue #50
New features: