Documentation needed for which coordinate system is used

[aaronfranke]

First of all, for Cartesian coordinates, which axis is "up", or perpendicular to the orbital plane? From looking at the code, I'm guessing that this is the Z axis, but this needs to be documented.

Secondly, what is the handedness of the coordinate system? Which direction is +Y relative to +X and +Z? If you look at the origin from (1, 1, 1), such that the XYZ unit vectors are all pointing towards you, in a right-handed coordinate system you see XYZ counterclockwise and ZYX clockwise, while a left-handed coordinate system has XYZ clockwise and ZYX counterclockwise. This also needs to be documented, perhaps on the same page as the above information.

Thirdly, it has been asked before if this library uses ECI or what, and it was stated that it uses J2000. This string only appears once in the documentation, so this needs to be clarified.