Merged requested to merge fixdt_kick into master
This fixes #146 (closed) and #103 (closed).
- 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.