Changed scenario to a test particle at the center of a unit sphere. The serial results are now compared against the vectorised results using the compare functions and differences are flagged.

Added functions to compare particles' properties and return an error if they differ more than the provided accuracy.

Replaced all vector reciprocals with macros and fixed a bug in the non-sym and sym vector force interactions. Missing grad-h multiply in the sph-term.

Added more vector intrinsics for AVX512 and support for left-packing vectors in multiple instruction sets.

Scheduler activate root

Re-set the task waits on activation, saves a loop over the tasks in `scheduler_start`.

See merge request !282

Added debugging checks to the pair and self tasks to make sure cells have been drifted to this point before they get updated.

Added debugging checks to the pair and self tasks to make sure cells have been drifted to this point before they get updated.

Added an (expensive) debugging check to verify that all cells have been drifted to the current point in time before a rebuild or a repartition.

Add -funroll-loops as a standard GCC optimization

SWIFT is faster with this.

Fixes #244.

See merge request !283

SWIFT is faster with this

Improve readability of active/inactive condition

This, as is, is rather minor but it's on the way towards improving the time-stepping. 

I have replaced all explicit checks that a particle or cell is active by calls to inlined functions. No change in functionality thus far.

The plan is to slowly introduce a better time-stepping mechanism where the particles carry less information (lower footprint) and make the whole time-step limiter problem easier. With this merge request in then I only need to make the changes in one single location (file active.h) and not everywhere. 

See merge request !279

Conflicts:
	src/runner.c

When not drifting because the cell has already drifted this step, keep the h_max and dx_max, don't reset to zero

Only one argument now

Makes sure that the hierarchical tasks get timed properly even when they recurse

Corrects #239 

See merge request !280