Fix outlet issue

CMake/cmake_config_files/MULE.config.cmake

apps/gpu/LBM/BoundaryLayer/BoundaryLayer.cpp

gpu.cmake

src/gpu/GridGenerator/grid/BoundaryConditions/Side.cpp

src/gpu/GridGenerator/grid/BoundaryConditions/Side.h

src/gpu/VirtualFluids_GPU/GPU/LBMKernel.cu