Change path module to functor; UnitPath replaces old Path
Add Path instance for Network_Common that uses the Int64.t costs on each link as the weight
Change name of Path to NetPath to avoid naming conflict with ocamlgraph
Add mutable weight field to allow reweighting edges for dijkstra's
Add edge_to_string function
path_functor: remove unused module
Merge pull request #46 from frenetic-lang/path_functor

Path functor
edges: expose find_all_edges operation

find_all_edges will return all edges that exist between two vertices.
The find_edge function only handles the special case where there is a
single edge between vertices and will raise an exception of no edge
exists. It will not return additional edges that may exist.