Only drift the particles that need drifting

Here is an algorithmic speed-up. The plan is to drift only the cells that contain an active particles and the cells that have at least one neighbour with an active particle.

• If this condition is not met then runner_do_drift() just returns without performing any operation.

• If the condition is met, runner_do_drift() moves all particles (as before) from the last point in time where a drift was performed.

We record for each cell the last time a drift was done such that we now by how much to drift things when needed.

Two exceptions to this rule: before reconstructing and before duping a snapshot, we drift all particles as we need correct position information at these two stages.

Note that I have made this an engine policy to ease the testing. Drifting all particles can be restored by setting the right flag.

@nnrw56 could you check that the function cell_is_drift_needed() makes sense and fully does what it says it does ? Also could you let me know whether you also agree with the whole logic and implementation.