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 `gadget2-part-update` branch should be merged with master prior to this merge request.

See merge request !204