Add a new CI job, which builds the VirtualFluidsCPU and treat all compiler warnings as errors.

As we want to remove all compiler warnings to improve our code quality, we need a CI job which ensures that no new compiler warnings are added to the repository. As the gpu part still has a lot of warnings, a first good start would be to remove all warnings in the cpu part.

A requirement for this issues should be, that VirtualFluidsCPU builds with the following compiler flags on GNU GCC:

-Wall -Wextra -pedantic -Werror