PymatchingDecoder.matched_edges(): Error finding the matched edge in the decoding graph

[hetenyib]

Steps to reproduce the problem

Look for the index of the boundary node/nodes and interpret the outcome of pymatching.Matching.decode_to_edges_array() accordingly.

What is the current behavior?

Currently it is assumed that there is one boundary node in the graph that is the last on in the list. This is not true e.g., for ARC circuits.

What is the expected behavior?

The method should return the correct DecodingGraphEdge objects regardless of the node index of the boundary node, and it should work for multiple boundary nodes.