Add limited support for hull

include/csg_types.hpp

src/csg/cgal_helper_polyhedron.cpp

src/csg/levelset_3d.cpp

src/csg/parser.cpp

src/csg/tests/CMakeLists.txt

src/csg/tests/levelset/hull.t.cpp

src/csg/tests/parser/hull.t.cpp

src/csg_cgal_helper.hpp