several changes: 1) remove init tasks and their dependencies, 2) init particles by hand in engine_init_particles, 3) make gravity tasks depend on ghost since inits are gone, 4) add hierarchical dependencies to sort tasks.

Conflicts:
	src/cell.c
	src/cell.h
	src/runner.c
	src/runner_doiact.h

remember at which step the cell was last sorted, enforce during drift that no sort is older than its sub-cell sorts.

Timebin optimization

This implements a change to the active/inactive checks following @jwillis' remark that the code was a bit slower than in the old master he was working on. 

The idea is to pre-compute the largest active time-bin and then use this information to decide whether act on particles or not. The old scheme was computing the end-time of the particle which involves a lot more information for the same result.

See merge request !316

Made all python scripts numpy 1.8.2 compatible, and fixed some bugs while doing it.

Title says it all. This should close #272.

See merge request !319