First attempt at a vectorised version of the symmetrical force interaction. Fixed signal velocity assignment in non-sym version.

Moved calculations for the Balsara switch and P_over_rho from hydro_iact_force and hydro_iact_nonsym_force into hydro_prepare_force and hydro_predict_extra.

Corrected divergence update with minus sign for vectorised versions of the symmetrical and non-symmetrical density functions.

Implemented vectorised versions of symmetrical and non-symmetrical density interaction functions for Gadget2.

Added place holders for the vectorised versions of force and density interactions for Gadget2 and Minimal SPH schemes.