title: Calculate sum of weights in NetworkX multigraph given path
body: <p>I want to get the sum of weights (total cost/distance encountered) of a given path in a networkx multigraph. </p>
  
  <p>It's like the current <strong>shortest_path_length()</strong> function but I plan to use it on the paths returned by the <strong>all_simple_paths()</strong> function. Is there a way to do that? </p>
  
  <p>I can't just iterate over all the nodes in the path because since it's a multigraph, I will need the key for that given path to be able to know which edge is used. Thank you.</p>