orbital mechanics stuff

- made Vector() able to accept a Vector as the input
- added subtractVector() to Vector()
- added getDistance() and getDirection()
- added getSemiMajorAxis()
- added getOrbitalParameters() [should work for particular cases but
otherwise returns NaN, but it SHOULD be correct when it works]
- added getApoapsis() and getPeriapsis() which are bullocks functions
and should almost never be used