As all warnings from VirtualFluidsCPU might be removed, we added a new CI job wich only builds VirtualFluidsCPU and treats all warnings as errors.

Increase the amount of ignored warnings for the lizard command, as gitlab CI cannot provide the lizard output when the job failed. Therefore we need this job to pass in any case. Also added the --warnings_only flag to make the output less cluttered.

Restructure the cmake cuda process. Remove the policy and always set CMAKE_CUDA_ARCHITECTURE to 30, if not set. This will fail when someone is trying to build with cuda 10.2 or higher, as cuda 10.2 minimum support is 52.

NOTICE: preferred way is to set CMAKE_CUDA_ARCHITECTURE as cmake argument OR in the config file to the real gpu architecture. For example phoenix tesla P100 have the compute capability 60.