Yen's k shortest paths iterator for edge weighted graphs

[markuspf]

I open this draft PR so that it is visible that I am working on an implementation of Yen's algorithm (https://en.wikipedia.org/wiki/Yen's_algorithm https://ia800704.us.archive.org/view_archive.php?archive=/24/items/wikipedia-scholarly-sources-corpus/10.1287.zip&file=10.1287%252Fmnsc.17.11.712.pdf) to iterate through paths in an edge weighted digraph in increasing weight order.

This draft PR relies on #656 .

I'll update the description when I turn this into a proper PR.