Make internal notation consistent

[moeyensj]

We need to re-standardize internal notation before the code gets to a stage where it will be harder to do so.

Propositon (following Milani et al. 2008):
r : heliocentric position vector in AU
q : heliocentric position vector of the observer in AU
rho : target to observer distance vector in AU
t : time in units of MJD

A suffix of _mag appended to a vector indicates the magnitude. A suffix of _hat indicates a unit vector.