title: pvd: time series with multiple datasets
body: <p>I am using <code>.pvd</code> file to load multiple files to Paraviw simultaneously. The file looks like this for time-series with a a single dataset:</p>
  
  <pre><code>&lt;?xml version="1.0"?&gt;
  &lt;VTKFile type="Collection" version="0.1"&gt;
    &lt;Collection&gt;
      &lt;DataSet timestep="0" file="a.1.vtu"/&gt;
      &lt;DataSet timestep="1" file="a.2.vtu"/&gt;
    &lt;/Collection&gt;
  &lt;/VTKFile&gt;
  </code></pre>
  
  <p>I would like to specify other files to be loaded along with <code>a.*.vtu</code> (e.g. <code>b.*.vtp</code>) which will show up as another item in the pipeline browser.</p>
  
  <p>I tried to put multiple <code>&lt;Collection&gt;</code> blocks to the <code>.vtp</code>, but Paraview loads only one of them. I also tried using <code>group</code> and <code>part</code> attributes to <code>&lt;DataSet&gt;</code>, without any result.</p>
  
  <p>How can I achieve having separate datasets (all of them saved at the same points in time) in one <code>.pvd</code> file?</p>