Replaced all calls to VEC_RECIPROCAL and VEC_RECIPROCAL_SQRT with vec_reciprocal and vec_reciprocal_sqrt.

Replaced all vector reciprocals with macros and fixed a bug in the non-sym and sym vector force interactions. Missing grad-h multiply in the sph-term.

Safer versions of the error() and message() macros that can be used in if/else statements without braces.

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

Moved the structure related to cooling into a separate file. Added a structure to the xpart to contain the accumulated radiative information.

Replaced all calls to fminf and fmaxf by the macros that was added for the integer min/max functions.