Add iact for star formation

In GEAR, we need to compute the gas' velocity dispersion for the star formation law. Thus we need to add a few computation in the density loop.

In this merge request, I am only implementing empty functions. My student will work on it during this semester, therefore it will be a bit long to wait on his work.

The structure is the same than in the chemistry and everything is implemented for EAGLE and the default star formation.