making RankedHandleGraph part of the default HandleGraph implementation #57
Description
In order to use the RankedHandleGraph interface on a PathHandleGraph, we need to either derive a new class, a RankedPathHandleGraph, or use a dynamic cast.
These are kind of common functions, and should have standard implementations that are basically trivial in all of our graph models. Maybe we would be better to just put them into the base HandleGraph class?