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.

CMakeLists.txt

@@ -103,13 +103,16 @@ if(BUILD_VF_GPU)
         message(FATAL_ERROR "CUDA Compiler was requested but is not found on the system.")
     endif()
 
-    if(NOT MSVC) 
+    if(MSVC) 
         # With the MSVC compiler we got this warning: nvcc : The -std=c++14 flag is not supported with the configured host compiler. Flag will be ignored.
-        # But we build the c++ code with C++14. Until we have not a solution here, we are not setting this on MSVC.
+        # But we build the c++ code with C++14. Until we have not a solution here, we set the standard to 11 here.
+        set(CMAKE_CUDA_STANDARD 11)
+    else()
         set(CMAKE_CUDA_STANDARD 14)
-        set(CMAKE_CUDA_STANDARD_REQUIRED TRUE)
     endif()
 
+    set(CMAKE_CUDA_STANDARD_REQUIRED TRUE)
+
     set(CMAKE_CUDA_FLAGS "${CMAKE_CUDA_FLAGS} -Xcudafe --display_error_number")
 
     if(NOT DEFINED CMAKE_CUDA_ARCHITECTURES)