Add write functionalities for Distributions in PrecursorWriter (not fully working, yet)

apps/gpu/LBM/BoundaryLayer/BoundaryLayer.cpp

src/gpu/VirtualFluids_GPU/GPU/CudaMemoryManager.cpp

src/gpu/VirtualFluids_GPU/GPU/CudaMemoryManager.h

src/gpu/VirtualFluids_GPU/LBM/LB.h

...rtualFluids_GPU/PreCollisionInteractor/PrecursorWriter.cu

...irtualFluids_GPU/PreCollisionInteractor/PrecursorWriter.h