Added new test that checks interactions between a pair of cells that have varying numbers of active particles.

Removed redundant tests from test suite. testPair.sh and testPairPerturbed.sh superceded by test27cells.sh and test27cellsPerturbed.sh. Added new testActivePair.sh that tests dopair1 when the number of active particles is varied.

Removed redundant tests from test suite. testPair.sh and testPairPerturbed.sh superceded by test27cells.sh and test27cellsPerturbed.sh. Added new testActivePair.sh that tests dopair1 when the number of active particles is varied.

Fixed seg fault bug. Make sure that max_index_i[ci->count - 1] and max_index_j[0] are initialised to correct values when nothing is in range.

Updated testInteractions to use new vectorised interaction functions, both using one and two vectors.

Conflicts:
	src/hydro/Gadget2/hydro_iact.h
	src/vector.h

Conflicts:
	src/kernel_hydro.h

Use intrinsics for vector operations as arithmetic overloading is not supported by the Intel compiler yet for AVX-512 vectors.

Conflicts:
	src/hydro/Gadget2/hydro_iact.h

Use intrinsics for vector operations as arithmetic overloading is not supported by the Intel compiler yet. Updated masking operations with new macros so that AVX-512 masking is supported.

Conflicts:
	src/kernel_hydro.h

Conflicts:
	src/vector.h

Use intrinsics for vector operations as arithmetic overloading is not supported by the Intel compiler yet.

Updated version number to 0.6.0 to reflect the large number of changes to the threadpool, drifts, MPI and scaling performance in the last few months.

Periodic gravity speed and accuracy improvements



See merge request !393