Fixes #74 by rewriting VertexRootLocator and introducing VertexSet.
Previous version used an other abstraction of vertices sets : the infamous VertexValueRange. Unfortunatly, resolving best possible VertexValueRange implied, by design, loading at least one full vertex value range.
By replacing these by VertexSet, I introduced a nice interface (`LazyLoader)` which sole purpose is to provide a list of vertices with a preloaded size.
I then used that LazyLaoder and a SortedSet to always explore the smallest available VertexSet. This is, inf act, a very nice improvement.