Replace shared_ptr by C++11 unique_ptr for multigons (single owner).
Use C++14 when available.  Allows make_unique.
Oops: if -> elif in SConstruct.
Use make_shared and make_unique where possible.
Fix typos.
Comment-out edge::attach_to_multigon (unused) with note about implementation.
Use nullptr where appropriate.
Re-order edge methods for clarity.