The private methods
- void writeCells2D()
- void writeCells3D()
- writePointsForMicroVertices()
- writePointsForMicroEdges()
are now members of a new VTKMeshWriter friend class.

The private methods
- writeEdgeDoFs()
- writeEdgeDoFData()
are now members of a new VTKEdgeDoFWriter friend class.

Makes enumeration vtk::DataFormat scoped for forward declarations