Changed call to part_is_active to new function. Also removed unnecessary load of v_hj in runner_doself2_vec.

Created a new function to check that a particle is active, without passing in a pointer to the engine.

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