Added CSV and VTK writer

src/CSVWriter.hpp

src/CSVWriter_unit_tests.hpp

src/GraphMLWriter.hpp

src/GraphMLWriter_unit_tests.hpp

src/VTKWriter.hpp

src/VTKWriter_graph.hpp

src/VTKWriter_unit_tests.hpp

src/VTKWriter_vector_box.hpp

vtk_graph_test.vtk