[WIP] Cooling : permit selection among various models at run-time

As we have a "contract interface" for the various physical parts (cooling, gravity, hydro, etc.) in the code I was wondering if moving to an implementation where the models are selected at run-time could be of interest.

I sketched the general idea using cooling as an example and opened this merge request to discuss the thing if it could be of interest (feel free to close if not). Will comment the code below to clarify a few points or highlight what could be controversial.