Unit tests for neighbour lists
Neighbour lists modules lack a unit test.
As lists are not directly comparable because they are not sorted, an N² algorithm that compares all pairs will be required. E.g making sure all particles in the neighbour list have a distance r_ij < r_c + r_skin, and all other pairs have a distance of r_ij > r_c + r_skin
Also edge cases should be considered (e.g. a cell list with 1x2x5 cells), two particle instances with highly different particle numbers.
This could also be used for (manual) performance tracking.