Kick task for fixdt and proper treatment of conserved quantities

This fixes #146 and #103.

 - 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.

See merge request !167