Disambiguate between node indices and (teir inner) petgraph indices.
Rename "band index" to "band first seg index", as we identify bands by
teir first segs. Rename "trianvertex index" and "navvertex index"
respectively to "trianvertex node index" and "navvertex node index", as
these point to nodes on the geometry graph and only indirectly to
vertices on the vertex graph.
This generally largely drawings from groupings. We need this because it
will be easier to handle all groupings on the level of layout as
continents and bands, which are groupings too, are to be tracked there.
There's a lot of rough edges when it comes to typing and contracts.