Kehl, a fast (no loop) method to compute MSD

Kehl computes the mean squared displacements (MSD), with no loop. Fully vectorized code.

MSD is a measure of the displacement over a certain interval of time, used for instance to analyze trajectories and tell if it's ballistic, diffusive, sub-diffusive, etc.
To compute MSD, all the possible time intervals within the trajectory need to be considerer. The simplest way uses nested loops :
the first loop runs over the time points, the other loop runs over the possible interval from each time point.

Thoses nested loops are a nightmare for fast computation, especially for long trajectories.

Kehl suppresses the loops, and computes all the squared distance at once, then sorts and averages them out.