BUG: Fix XdmfTopologyConverter to perform node insertion solely
based on connectivity. Previously, nodes added to a mesh in XdmfTopologyConverter were compared using position to ensure nodes added to element boundaries were not duplicated (added more than once). This failed in some cases, especially for highly refined meshes, because of incorrect floating point comparisons. Now, shared nodes are identified using connectivity, ensuring that nodal addition should work in all cases, even for highly refined meshes.