Logger example

Hey,

In this merge request, I am adding a new example for the logger. It is a simple planetary system (on circular orbits) around a star, then I am comparing the energy between the usual output and the logger. In the figure, I am plotting the error on the energy (E - E(t=0)) / E(t=0) (should I change the ylabel with that?) for the snapshots (blue dots), the statistics file (red line), the logger without interpolation (orange crosses) and the logger with interpolation (green line).

In order to do this example, I have implemented a new python function that move all the particles forward in time and also two functions for the Hermite interpolation (position and velocity).