reading higher-order elements with ContinousGridCreator results in additional vertices
When reading a file with higher-order elements using the default
ContinousGridCreator, all vertices found in the file are inserted into the GridFactory even those not used. Subsequently writing the grid to a file with VtkWriter yields a file with many additional vertices all at coordinates (0,0,0).
The simplest fix is to only cache the vertices in
insertVerticesImpl and insert them into the factory later in
insertElementsImpl selectively just like
LagrangeGridCreator does it.