Orbital dynamics calcuations

[mkelley]

High-level concept
Add methods to Orbit to calculate the Tisserand parameter with respect to another body, and the orbital similarity parameter D (Jopek 1993).

Explain the relevance to sbpy
These are standard solar system parameters that are easy to implement.

Example (pseudo-)code

comet_252p = Orbit(...)
comet_BA14 = Orbit(...)
Jupiter = Orbit(...)
T_J = comet_252p.tisserand(Jupter)
D = comet_252p.d_criterion(comet_BA14)