Test periodic bc

This tests the periodic boundary conditions which are generated in `space_getsid()` and used in `runner_dopair1_density`.

An 8x8x8 mesh is generated that is periodic and 26 pair interactions are computed for the following cells:

* All 8 corner cells
* The cell at the centre of each face
* The cell at the centre of each edge

These checks are performed for both unperturbed and perturbed cells.

See merge request !350

Output the position of the cell in the output of the periodic boundary condition test. Better tolerance requirements.

Corrected mistake in the periodic boundary test that prevented compilation when optimisations are switched off.

Make the difffloat script use a new line in the files to set a limit below which values are ignored as too close to zero.

 - Use the correct value of 'h_max' in the construction of the caches in the vectorized DOPAIR1 routine. (fix #312)
 - Update the test27 unit test to also randomly perturb the smoothing length values.
 - Set a correct value for h_max in the test27 case and not just a loose upper-bound.
 - Call the non-vectorized version of DOPAIR1 for the corner interactions.

See merge request !348

EAGLE_25 now produces the same number of time-steps whether running with or without vectorization. 

Documentation update. The MPI and vectorization macros will be expanded when generating the doxygen files.

Moved the testing into a function and tested each corner cell and each cell at the centre of each face.

don't re-sort stale dimensions on cleanup as we don't know if we're going to need these at all. they will get resorted if and when needed.

change sort task flags to be zero by default and only switch on those that are needed, avoids pre-emptively computing all the sorts for a cell since they may never be needed between two sort cleanups.

Conflicts:
	src/engine.c
	src/runner.h
	src/runner_doiact.h
	src/scheduler.c