add sample test objects and rebound code

[tamidodo]

Adds two functions:

• one that converts state vectors from ECI epoch of state equatorial coordinates in m and m/s to solar system barycentric J2000 ecliptic coordinates to be compatible with Rebound simulations
• another one that sets up a simulation with the main solar system bodies and the object of interest for a variable amount of time, in a given time direction (forwards or backwards time), and returns a text file containing the orbital elements at the end of that period.