Added merge() to function module.
slope_triangle gives warning instead of raising Exception when slope is inf/nan.
Make sure identical elements (as compared by strings of their vertices) get same hash.
Orientation bug fix in singular quad scheme (and update unit test).
Remove commented Topology.trim code.
UnstructuredTopology.groups is now a dict, for consistency.
Added Topology.glue (and its unit test).
Subdivision surface capabilities: catmull-clark element, blender geometry import, unit test