Self-inclusion in topology information incomplete

[termi-official]

The new ExclusiveTopology returns an incomplete neighborhood when asking for self-inclusion of the element (e.g. getneighborhood(top, grid, cellidx; include_self=true) - this case is not covered by our unit tests https://github.com/Ferrite-FEM/Ferrite.jl/blob/master/test/test_grid_dofhandler_vtk.jl#L313-L338 .

Also I notices two undocumented convenience functions https://github.com/Ferrite-FEM/Ferrite.jl/blob/master/src/Grid/grid.jl#L395-L396 , maybe we could add these to the docs or are they for purely internal use?

cc @koehlerson