Crash when compiling with grackle_1 2 or 3 but running without --cooling

In the current implementation, we can compile with grackle_0 and run a simulation without --cooling. It works fine. However, if we compile with grackle_X, X > 0, the code crashes without saying why...

Debugging shows that the code crashes in cooling_first_init_part() because it calls an uninitialised grackle_data object. The data are initialized in cooling_init_backend(). However, since we are running without cooling, this function is not called.

The solution would be to pass the information of whether we run with cooling to cooling_first_init_part() and return if we run without cooling.