VTKSequenceWriter should expose vtkWriter_->clear()

Since addCellData and addVertexData of the internal VTKWriter are exposed through the VTKSequenceWriterBase parent class, it would be consistent to also add the clear function to this interface. It saves the user of VTKSequenceWriter from dragging around another shared_ptr to the underlying VTKWriter to clear unwanted VTKFunction references.

The implementation of this is trivial, so if you agree that this should exist, I'd be willing to create a pull request providing this.