Fixed indentation.
It is possible to output region ids to VTK file format.

* Outputing region IDs is optional.
* When you want to add region IDs to output file, then modify
  configuration file like this:

  system = {
    output_stream = {
        file = "./test1.pvd",
        format = {
          TYPE = "vtk",
          variant = "ascii",
          region_ids = true
        },
        name = "flow_output_stream"
      }
  }