Fix Grackle cooling MULTI_COOL error with metal

With @yvesrevaz we encountered a problematic issue with grackle. When the gas density exceeds the maximal density tabulated in grackle Cloudy tables, grackle extrapolates the values. This extrapolation leads to some MULTI_COOL errors (within grackle). The internal energy does not converge and its derivative oscillates.

To fix this problem, we set a new maximal density for cooling that is passed to grackle. Then, everything is fine.

I updated the documentation as well to explain this problem in more detail.