cooling only works with CGS unit system

[BenWibking]

Describe the bug
The cooling module assumes that the problem uses UnitSystem::CGS. Problems that use other unit systems will have the wrong cooling rates applied.

Additional context
N/A

[chongchonghe]

Any problem with this? We could add a flag to throw an error at compile time when using the cooling module with other unit system.

[BenWibking]

Yeah, I think that's the solution for now.

[BenWibking]

This is forbidden at compile time now, so I'll close it.