The lru cache on is_connected causes invalid results if new edges are added

[fiendish]

Adding new edges has to clear the cache, otherwise you get:

>>> from graph import Graph
>>> a = Graph()
>>> a.is_connected(1, 2)
False
>>> a.add_edge(1, 2)
>>> a.is_connected(1, 2)
False

[fiendish]

Also it's not documented that is_connected has an lru cache on it, so one would never know that the cache needs to be flushed unless examining the internal code.

[root-11]

+1. Shall we remove it? Or declare it?

[fiendish]

Probably remove it. If someone wants to cache results, they can always do it externally.

[root-11]

phaselines requires some form of caching to work effectively. Proposal is in branch lru_cache. Please review.

[fiendish]

I added some comments on the branch.