GeometryGrid: use `hasGeometry` and `hasEntityIterator` instead of `hasEntity`
Grids are required to provide subentities for all codimensions.
GeometryGrid in addition provides iterators and entity geometries for all codimensions (even those not provided by the host grid).
It should use
hasGeometry (#14 (closed)) and
hasEntityIterator (to be implemented) instead of the deprecated
hasEntity capability to decide whether to just wrap the host grid implementation or to provide its own "fake" implementation.