Kick task for fixdt and proper treatment of conserved quantities

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.

Added 1 commit:

• ba4e72b1 - Documentation and formatting

Added 1 commit:

• de688b82 - Simplified drift

Seems to compile and run correctly for me, so accepting. Please remove the branch if not needed.

mentioned in commit ebc1a2a5

Status changed to merged