RiverGraph redesign

[jkibele]

So, the RiverGraph object currently includes the coastline because it needs a concept of "river mouth" that is distinct from just a dead end. It might be cleaner to, if possible, use GraphBuilder to generate a RiverGraph that is pruned of any extraneous dead ends. Then RiverGraph could treat all deadends as river mouths in distance calculations.

Essentially, I think it would make more sense to have the RiverGraph object only do river related things (like calculate the distances).

[jkibele]

I've started working on this now, and I've decided to do it differently. I've decided to make the coastline attribute optional. I've also decided that I don't need to use GraphBuilder to prepare a RiverGraph.