Remove all warnings in VirtualFluids CPU (Closes #6, #7)

Closes #6 and #7

See merge request irmb/VirtualFluids_dev!20

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

 - metis: do not pass -O3 flag on msvc compiler, as this flag is ignored anyway. Instead of /w pass w0, which is equivalent to /w.
 - RheologyK17LBMKernel: A and B are also defined in D3Q27System.h. Therefore to not hide this declaration they are renamed to A_ and B_
 - additional small clean ups...

Feature/cleanup vf cpu (Closed #3)

Closes #3

See merge request irmb/VirtualFluids_dev!19

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.

Feature/thixotropy

See merge request irmb/VirtualFluids_dev!18