New time line

Changes include:
 - Time line is now based on a `long long` rather than `int` for more flexibility.
 - Particles only carry a `char` to indicate their time bin rather than two `int` for 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.

See merge request !301

Buffered cell_split

This "works", but my tests crash because of #248, which I get every time I run EAGLE_12 with a single thread.

@pdraper, can you give this a spin through your regular tests to see if there are no hidden bugs/problems I've missed? Thanks!

See merge request !294

change space_getcell to space_getcells, can pick up more than one cell at a time.

As discussed in #255. Have only checked that it works, haven't done any benchmarks, but this can only make things faster 😉 

See merge request !300

Call vector

Call vectorised version of `runner_doself1_density` when vectorisation is turned on.

See merge request !302