Edge weights not being passed to igraph::shortest_paths()

[jmlondon]

prt_shortpath() passes NULL to the weights argument of igraph::shortest_paths() and this results in the shortest path being determined based on the fewest number of nodes instead of distance.

the fix is to use the sfnetwork::set_path_weights() to create the vector of weights and pass that instead of NULL

[jmlondon]

this original hypothesis might not be correct ... after @dsjohnson did some testing, it seems igraph might have been using the weight column in the vis graph after all. Regardless, still probably best to be explicit and set the weights.