We can now export all of the following functions
- P[1,2]Function
- P[1,2]VectorFunction
- EdgeDoFFunction
- DGFunction
for all of the following value types
- double
- int32_t
- int64_t

Commit extends VTKOutputTest to check that this works and also
makes some details of the implementations in VTK*Writer classes
more consistent.

Occurences of double and "Float64" get replaced by real_t and
typeToString< real_t >().

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

Having DoFType as private enum inside VTKOuput will not work together
smoothly with the VTK*Writer friend classes when we need to pass a
DoFType. E.g. we cannot forward declare it. So the commit makes it an
enum inside the vtk namespace instead.

Commits places the following free functions in the new file VTKHelpers.hpp
- writeXMLHeader()
- writeXMLFooter()
- writePieceHeader()
- writePieceFooter()
They now belong to the namespace vtk.

We also convert VTKOutput::openDataElement() into a free function, placed
in the same file and namespace as above.

Additionally we remove the class enum VTKOutput::VTK_DATA_FORMAT and replace
it by vtk::DataFormat.

The private methods
- writeP1()
- writeVertexDoFData()
- writeP1VectorFunctionData()
are now members of a new VTKP1Writer friend class.

The private methods
- writeP2()
- writeSingleP2Function()
- writeSingleP2VectorFunction()
are now members of a new VTKP2Writer friend class.
The private method
- writeDGDoF()
is now a member of a new VTKDGDoFWriter friend class.

Finally we modify the VTKOutputTest so that some data are actually written
in base64 encoded binary