Intrinsic vectorisation

I have implemented vectorised versions of the symmetrical and non-symmetrical density and force interaction functions for the GADGET_SPH scheme.

There are also tests for each interaction in tests/testInteractions.c.

There are some bug fixes in the DEFAULT_SPH along with changes to the struct part and minor renaming of variables.

Calculations of the balsara switch and P_over_rho2 are now performed in hydro_prepare_force() rather than in runner_iact_nonsym_force or runner_iact_force.

Note: The !203 (merged) gadget2-part-update branch should be merged with master prior to this merge request.