Support giving orbits as two lengths rather than eccentricity #52
Description
Stock comets are usually specified by constraining their periapsis and apoapsis. This system allows much better control of highly eccentric orbits than the Custom Asteroids approach of constraining the eccentricity and one length (periapsis, apoapsis, or semimajor axis). As a result, Custom Asteroids comets can't show as much diversity or consistency as stock comets.
Provide a config block that lets comets constrain two different lengths, and ensure it interoperates with all existing config styles. Providing the same type of length twice, or two lengths and an eccentricity, should be rejected as an error.