extended utility entities

[niniemann]

This PR adds some new functionality I needed while working on a ROS wrapper for sempr:

• SpatialObject (RDFPropertyMap + GeometryCollection)
• Geometry::clone()
• optional functor for ObjectQuery

  auto q = std::make_shared<ObjectQuery<Person> >(
    [](Person::Ptr person) { return person->name() == "Max Mustermann"; }
  );

• RDFDocument loads triples from a file

I guess there will be more, and not everything will be useful in general... But I think the functor for object-queries and the RDFDocument are worth a PR.

[niniemann]

A bit less patient, but I'd like to clean this mess up a bit. And with "mess" I mean open PRs. :D