Improve route code

[lverweijen]

Some simplifications to Route

• Add more unit tests.
• Handle some edge-cases correctly. Mostly when route has only 1 of 0 anchors.
• Always store route._ancestors_levels instead of calculating these lazily (a form of premature optimization).
• Merge Route and NodesView classes (simplify api). Make route.nodes return route for backwards compatibility.
• Make anchors return a tuple instead of an AnchorView. There are typically only a few anchors, so no need to make this more complicated than needed.
• Make reversed(route.edges) return edges in parent-child order for consistenty with iter(route.edges).

[lverweijen]

Some ideas for later:

• Make node.path.edges work. This would provide a nice consistency with route.edges. node.path could even implement some kind of BaseRoute interface.
• Make route.add_anchor(anchor) accept a second optional argument in which a precalculated path can be given.
• Make new_route = Route(old_route) work. It should recalculate the route. It would be the same as new_route = Route(*old_route.anchors). This can help if the tree changed in the mean time.
• Make route = Route(node.path) work, except I don't know if a Route object has much benefit over using a list.
• Make route_combined = route_a + route_b work. The benefit over a list is that this should connect the endpoints of route_a and route_b in the result. Multiplication could also work. It seems intuitive to me, but I don't see many cases where this can be used.
• Make route.reverse() work. Easy to implement, but no clear benefit over just storing the route in a list and reversing that.

Edit: Thinking some more about addition. If implemented, it should probably require the endpoints to be the same, so route_a should end where route_b begins. Otherwise, raise ValueError. Then forget about multiplication.