Adding SparseGridGpu iterators

src/Grid/grid_base_implementation.hpp

src/Grid/grid_common.hpp

src/Grid/grid_key.hpp

src/Grid/grid_sm.hpp

src/Grid/iterators/grid_key_dx_iterator_sub.hpp

src/Grid/map_grid.hpp

src/SparseGrid/SparseGrid.hpp

src/SparseGridGpu/BlockMapGpu.hpp

src/SparseGridGpu/Iterators/SparseGridGpu_iterator.hpp

src/SparseGridGpu/Iterators/SparseGridGpu_iterator_sub.hpp

src/SparseGridGpu/SparseGridGpu.hpp

src/SparseGridGpu/SparseGridGpu_ker.cuh

src/SparseGridGpu/performance/Stencil_performance_tests.cu

src/SparseGridGpu/tests/SparseGridGpu_tests.cu

src/SparseGridGpu/tests/utils/SparseGridGpu_testKernels.cuh

src/Vector/map_vector_sparse.hpp

test_data/SparseGridGPU_output3D_test.vtk