Changed scenario to a test particle at the center of a unit sphere. The serial results are now compared against the vectorised results using the compare functions and differences are flagged.

Move rho_dh into the density sub-structure. Update the pressure and sound-speed after kick and drift

GCC 5x doesn't like this now we have -Wextra, if offends make count size_t as well

Created function pointers to test all different types of interactions: density (non-sym/sym); force (non-sym/sym) for serial and vectorised versions.