Refactor half edge mesh and tmesh to become one class

[jovobe]

• store knots and corners directly in half_edges
• Ensure each face has a linked edge, which is based at a corner
  • h(f) should return a half edge, which is based at a corner. This is currently the case by coincidence and should be guaranteed.
• Some constraints of a normal HEM are not supported. We need to check for them and fix them. The known constraints are:
  • Manifold edges
  • Fan blades
  • Fan objects
  • Orientability
  • Same vertices face in reverse order
  • Given {v0, v1, v2, v3, v4}, a face with v0–v3 already exsits, what if we want to create another face with v0–v4?
• Rethink use of hem::get_next -> this could be removed.
• C.5 while (g != h) -> refactor to use get edges in face.
• zero knot intervals!
• check for the following things:
  • rule of 5 on every type
  • const correctness
  • docs
  • are the methods implemented in a performant way?