Add distance and pointing angles

The trajectory should provide an easy way to compute the distance and the angle between one of the spacecraft axis and a target body:

traj.target_distance('JUPITER', unit='AU')

traj.target_angle('JUPITER', axis='+Z')

This should allow the user to perform this type of plot:

The pointed target does not require to be the same as the primary target.

In both cases, we need to compute the sc -> target position vector in J2000 frame. This value should be cached internally to avoid to compute it multiple times.