Add tests for GridImp::setStopperNeighborCoords();

src/gpu/GridGenerator/CMakeLists.txt

src/gpu/GridGenerator/grid/Field.cpp

src/gpu/GridGenerator/grid/Field.h

src/gpu/GridGenerator/grid/GridImp.cpp

src/gpu/GridGenerator/grid/GridImp.h

src/gpu/GridGenerator/grid/GridImpTest.cpp

src/gpu/GridGenerator/grid/NodeValues.h