BUG: vtkCloud suppresses output for empty clouds (closes #866)

- the original intention was to avoid vtp output when the clouds are
  empty anyhow. However, it is useful for post-processing to have
  clouds with zero parcels (eg, before the start of injection).

  Pruning of empty clouds is now an option in the vtkCloud dictionary
  controls, with the default being false (no pruning). This represents
  a non-breaking change in behaviour since it generates more output
  than previously.

  Example,
  {
      type    vtkCloud;
      //- Suppress writing of empty clouds (default: false)
      prune   true;
  }