Function to assign flows to network edges

[luukvdmeer]

Given an OD matrix of flows between nodes, and potentially maximum capacities of edges, it would be nice to be able to assign flows to each edge using flow assignment algorithms. Just as in routing, we could interface here to different backends, as both dodgr and cppRouting already seem to have functionalities for this. One way to integrate this into the existing design could be a edge_flow() function in the same style as the edge measure functions such as edge_length() and edge_circuity().