Modified PointData and CellData VTK export

[JonasBreuling]

This pr adds the possibility to export BEZIER curves and cell as *.vtu files. Visualization via paraview requires special treatment of the PointData and CellData fields RationalWeights and HigherOrderDegrees, respectively, see paraview issue.

For both fields the corresponding *.xml files have to be of the form

<PointData RationalWeights="RationalWeights">
  <DataArray type="Float64" Name="RationalWeights">
    1 
    1 
    1 
    1 
    0.70710678118
    0.70710678118
    0.70710678118
    0.70710678118
    1
  </DataArray>
</PointData>

and

<CellData HigherOrderDegrees="HigherOrderDegrees">
  <DataArray type="Float64" Name="HigherOrderDegrees" NumberOfComponents="3">
    2 
    2 
    0
  </DataArray>
</CellData>

This pr

1. adds the cell types "VTK_BEZIER_CURVE", "VTK_BEZIER_TRIANGLE", etc.
2. passes the PointData and CellData names to exported *.xml files.

[JonasBreuling]

Strangely enough, the black test fails. No error message appears on my local machine.