Allow OrbitPhase to be specified as a radius/time #49
Description
Currently, the OrbitPhase block can be given as either mean anomaly or mean longitude. However, this makes it hard to control the discovery phase of an object if it has a range of eccentricities, especially if those eccentricities are close to 1 (either eccentric or hyperbolic).
KSP's stock comets constrain the discovery phase as the radius at which the comet is first detected. It would be useful if OrbitPhase had a similar capability, with the user able to give a minimum or maximum allowed radius (this would, of course, need to be validated against the orbital apses). I'm not sure how the user should distinguish between incoming and outgoing phases.
Another useful parametrization for OrbitPhase would be as an absolute time. This would have no ambiguity, since it could follow the mean anomaly convention of negative values for phases before periapsis and positive values for after.