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