Consider namespaces

This is intended as a discussion-issue first.

Do I see it correctly that we don't consider namespaces at the moment at all?

The `gml` namespace may be bound to absolutely anything, but we'll still happily consider `gml:Polygon` to be a GML element. Same for attributes.

What's worse, it is perfectly valid (and actually quite popular) to declare the primary namespace as default namespace (`xmlns="http://www.opengis.net/gml"`). In this case `Polygon` will belong to the `GML` namespace - without any prefix. If I see it correctly, we won't process such elements at all then.

Let's discuss what do we do about this.

No file or test entry point is named. Start by reviewing the GML polygon parser's handling of prefixed and default namespaces, then clarify the expected behavior for namespace-bound elements and attributes. Done means reaching agreement on the namespace rules and defining coverage for both prefixed and default GML namespaces.

Written by the indexing model from the issue text.