Bw-graph-tools

This release is a big shift from purely functional programming to storing state in the graph traversal class instances. Storing state was necessary to support `SameNodeEachVisitGraphTraversal`, which needs to remember when a node has been visited.

The old API still works but is deprecated; please shift from `NewNodeEachVisitGraphTraversal.calculate()` to `NewNodeEachVisitGraphTraversal().traverse()`. Note that the graph traversal input configuration is now wrapped in a Pydantic class `GraphTraversalSettings`.

* [21 Clean up changes from shift to stateful calculations](https://github.com/brightway-lca/bw_graph_tools/pull/21)
* [20 Add `SameNodeEachVisitGraphTraversal`](https://github.com/brightway-lca/bw_graph_tools/pull/20)
* [17 Major documentation upgrade](https://github.com/brightway-lca/bw_graph_tools/pull/17)

* Fix [16](https://github.com/brightway-lca/bw_graph_tools/issues/16) - Add optional `max_depth` argument to graph traversal

Added the following attributes to `Node` dataclasses:

* direct_emissions_score_outside_specific_flows (float): The score attributable to *direct emissions* of this node which isn't broken out into separate `Flow` objects.
* remaining_cumulative_score_outside_specific_flows (float): The *cumulative* score of this node, including direct emissions, which isn't broken out into separate `Flow` objects.
* terminal (bool): Boolean flag indicating whether graph traversal was cutoff at this node

* Fix [18 - Duplication of values from first heuristic](https://github.com/brightway-lca/bw_graph_tools/issues/18)
* Update packaging
* Python 3.9 compatibility

* Remove `scikit-network` dependency

* Updated CI scripts