VelocityOutput doesn't work correctly for Richards

Bug report

What happened / Problem description: When running a Richards test with vtkWriter.addVelocityOutput(....); and AddVelocity = true in params.input Paraview crashes while loading the files, because the gas velocity is -nan.

What you expected to happen: The gas velocity should not be written out for the Richards model.

How to reproduce it (as minimally and precisely as possible): Add [Vtk] AddVelocity = true to a Richards test i.e. richards/nonisothermal/conduction/params.input