Merge branch 'testPeriodicBC' into 'master'

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