[bugfix,documentation] Fix doxygen grouping of public methods
(cherry picked from commit 39d285a65601d85595d3d85bdd619025dc5576af)

Signed-off-by: Christoph Grüninger <gruenich@dune-project.org>
[bugfix,documentation] Minor documentation improvements
(cherry picked from commit 31fe278ee209bf753e35d3640e0de8c2e00f7639)

Signed-off-by: Christoph Grüninger <gruenich@dune-project.org>
Mark the exported types {Leaf|Level}IntersectionIterator as deprecated

They were needed as return values for the i{leaf|level}begin and i{leaf|level}end
methods, but those have been deprecated a while ago.  Hence it makes sense
to deprecate the types as well.

Please merge this to the 2.3 release branch.
(cherry picked from commit d57f5aae82de79c6054a6674d9c825d3e903ee8a)

Signed-off-by: Christoph Grüninger <gruenich@dune-project.org>