adds constant velocity to precursorBC

...GridGenerator/grid/BoundaryConditions/BoundaryCondition.h

src/gpu/GridGenerator/grid/GridBuilder/GridBuilder.h

src/gpu/GridGenerator/grid/GridBuilder/LevelGridBuilder.cpp

src/gpu/GridGenerator/grid/GridBuilder/LevelGridBuilder.h

...tructureInitializer/GridReaderGenerator/GridGenerator.cpp

src/gpu/VirtualFluids_GPU/GPU/GPU_Interface.h

src/gpu/VirtualFluids_GPU/GPU/GPU_Kernels.cuh

src/gpu/VirtualFluids_GPU/GPU/LBMKernel.cu

src/gpu/VirtualFluids_GPU/GPU/PrecursorBC.cu

src/gpu/VirtualFluids_GPU/KernelManager/LBKernelManager.cpp

src/gpu/VirtualFluids_GPU/LBM/LB.h