Consider allowing geometry fields through foreign key relations

[stewjb]

It seems like features can only be generated with querysets from models that have embedded geometry fields. There are times where it makes more sense to start with another model and access the geometry field through a foreign key relation.

[vdboor]

Thanks an interesting idea, and I'm all for supporting that.

This change will no doubt bump into a lot of issues but they are totally solvable within the current design (e.g. the FeatureType / XSD type abstractions)

I do would like to ask if you could write a PR for this. This isn't something we're expecting to need in the near future.