Mesh connectivity tests for line and quad meshes.

Related to the https://github.com/ufz/ogs/pull/185 I wrote several basic tests checking mesh's elements and nodes connectivity. While writing the code I found two issues:
 - `MeshLib::Element::getNNeighbors()` function is *not* returning the number of neighbors but something else most probably the (default) maximum of neighbors of an element.
 - The line elements cannot have neighbors -- it is statically set to zero!!!

Pull-requests correcting the issue are welcome.