make tracer return positions and velocities as separate arrays

[jobdewitte]

Problem Statement

Currently the tracer returns the trajectories as (N,6) dimensional arrays named positions with the first three elements along the second axis representing the (x,y,z) coordinates and the later three the velocities (vx, vy, vz). I reckon this is fairly confusing.

Proposed Solution

Split the aray in two (N,3) arrays representing positions and velocities separately.