Removed alternate version of doself2_vec that was slower than current version. Renamed some variables. Removed left-packing functions for force interactions as they are not used anymore.

Created new version of doself2_vec that computes force interaction on the fly, 2.1x faster than serial.

Conflicts:
	src/cache.h
	src/hydro/Gadget2/hydro_iact.h
	src/kernel_hydro.h
	src/runner_doiact_vec.c
	src/runner_doiact_vec.h
	src/vector.h
	tests/benchmarkInteractions.c
	tests/testKernel.c

Cache auto vec

Auto-vectorise populating the cache with hints to the compiler and `restrict` keyword.

See merge request !404

Correct the use of rshift to reduce the number of distance calculations.

Applies `rshift` (boundary conditions) correctly in `DOPAIR1` + `DOPAIR2` to reduce the number of spurious distance calculations.

See merge request !403

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* Removes testPair and testPairPerturbed which are superseded by test27cells and test27cellsPerturbed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

Updated tolerances because the command line argument, -p, is now used in test27cells.c to perturb the smoothing length and not just the default value of 1.1f.