In brief this adds a new task "kick_fixdt" for the case where we run with this policy.
We then construct the type of kick task corresponding to this policy.
Some of the physics operations have been moved to separate files "drift.h", "kick.h" and "timestep.h" to simplify things and prevent redundancies.
Energy statistics are now computed at the end of the drift (and not of the kick any more) which allows to get a value even in a time-step not involving all particles.
The frequency of the statistics is specified in the parameter file.