NCMesh Tree Traversal API

[termi-official]

I think we are missing some API to traverse non-conforming meshes. Especially accessing information of the refinement tree is currently not really possible. Having such an API is interesting for several practical applications. To give two concrete examples:

1. AMR Multigrid: There are constructions of specialized multigrid preconditioners and solvers, which abuse information on the refinement hierarchy. This is currently just partially possible with the FESpace hierarchy class and not all techniques work out properly.
2. Multi-Mesh AMR Techniques: For coupled multiphysics problems it can be beneficial if the individual solution fields are independently refined for each other. However, it is still required to resolve the integrals correctly. E.g. the trial basis functions can be on a coarse mesh, while the test basis functions are on a fine mesh. Currently we can transfer information (e.g. integration point evaluations) with GSLIB, but this is not super efficient and the NC mesh basically has all the information available. Efficient algorithms exist here, e.g. "dual mesh traverse".