Refactor VectorFieldVTKWriter and VectorFieldGnuplotWriter
The VectorFieldVTKWriter and VectorFieldGnuplotWriter classes should be refactored analogically to MeshFunctionVTKWriter and MeshFunctionGnuplotWriter.
The VectorFieldVTKWriter and VectorFieldGnuplotWriter classes should be refactored analogically to MeshFunctionVTKWriter and MeshFunctionGnuplotWriter.