Merge branch 'AVX512-Fixes' into 'master'
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generic mask, `mask_t`, for each instruction set (AVX, AVX2, AVX512) * Removes testPair and testPairPerturbed which are superseded by test27cells and test27cellsPerturbed * Adds testActivePair that computes the density between a pair of cells in various configurations of active particles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