Add option to adjacency matrix functions to adjust parallel edge behavior

[mtreinish]

What is the expected enhancement?

Right now the adjacency_matrix() function unconditionally sums the weights for parallel edges in the graph. We should make this configurable so that we can do different operations. Off the top of my head, sum(), min(), max(), avg() are all good candidates.

For maximum flexibility we could just make this a python callback function that get's passed both weights and returns the f64 to use (which is what I think networkx does). However that will have much worse performance than using a f64 operation in rust.

[danielleodigie]

Can I be assigned to this?