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

Changed 'c' and 'POrho2' to 'soundspeed' and 'P_over_rho2'. Fixed signal velocity assigment bug in vectorised versions of the force interactions.

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.

Rearranged and replaced: div_v,curl_v->balsara,entropy_dt,pressure->P_over_rho variables of the part struct.

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.

Removed all compiler macros for vectorisation and replace with HAVE_ macros. Added WITH_VECTORIZATION #ifdef for turning vectorisation on and off.