Added functionality for weighted graphs and node / edge deletions.
Added functionality for weighted graphs and node / edge deletions.

* Weights are stored sparsely and a default weight may be set (e.g. 0).
* Added weight store member to Graph class.
* Added DictWeightStore implementation.
* Added ability to sort when dumping graph with `to_nodes_edges` or
  `to_nodes_edges_weights`.

afbarnard/barnapy