Remove GPU compiler warnings (Closes #4, #12)

With this merge request, all compiler warnings for VirtualFluids_GPU are removed. This includes:

• gcc -Wall -Wextra -pedantic
• clang -Wall -Wextra -pedantic
• msvc /W3

But we should keep in mind that there are still a lot of warnings suppressed for these three compilers. We should get rid of them one day.

The CI Script build jobs were also adapted in a way, that all compiler warnings are treated as compiling errors. Therefore we should not get any new warnings into our code.