Expose widest paths functions

[szhorvat]

Expose widest paths functions as widest_paths() (mirroring shortest_paths())and widest_path_widths() (mirroring distances())

There's a problem to solve when doing this: The weights parameter of the corresponding C functions is not optional. In R, it must be optional to be able to signify that weights are to be taken from attributes. A proper setup that makes this easy to do is not ready yet.