Avx512 fixes

* Adds a generic mask, `mask_t`, for each instruction set (AVX, AVX2, AVX512)
* Removes testPair and testPairPerturbed which are superseded by test27cells and test27cellsPerturbed
* Adds testActivePair that computes the density between a pair of cells in various configurations of active particles (face, edge, corner)
* Replaces all arithmetic vector operations to support AVX512 as there is no compiler support to overload them
* Computes max_index_i[] for each particle in runner_dopair1_density_vec to follow ParCo paper
* testInteractions now calls correct vectorised density particle interactions and is run as a part of the test suite
* Solves #327


See merge request !396