Revisit the relation assembly process

[jpolchlo]

From a comment on #60 by @mojodna:

I want to revisit the relation assembly process (for non-multipolygons) to prevent new geometries from being created and just propagating relation tags onto the constituent lines / polygons so that they can fulfill the same role without duplication. It does complicate things a bit, resulting in more minor versions of ways, as changes to relations that they're part of will change their metadata (similar to how changes to nodes change their geometry.

This should be addressed at some future point.