STYLE: restrict vtkCloud output names to use time-index (issue #866)

- this provides easily sortable names.

  The time information is available via the corresponding .series
  file, which can also be loaded directly with ParaView 5.5 or later.

  For further redundancy, time information is saved in the first XML
  comment inside of the .vtp files. For example,

      <!-- cloud=limestoneCloud1 time=0.005 index=200 -->