New time line
Changes include:
- Time line is now based on a 
long longrather thanintfor more flexibility. - Particles only carry a 
charto indicate their time bin rather than twointfor their start and end of time-step. - The kick task is not split in kick1, kick2 and timestep.
 - When compiling with the debugging checks, we verify that the particles have been properly drifted and kicked before any interaction.