title: Partition data coming from CSV so I can process larger patches rather then individual lines
body: <p>I am just getting started with Google Data Flow,   I have written a simple flow that reads a CSV file from cloud storage. One of the steps involves calling a web service to enrich results. The web service in question performs much better when sending several 100 requests in bulk.  </p>
  
  <p>In looking at API I don't see a great way to aggregate 100 elements of a PCollection into a single Par.do Execution. The results would need to be then split to handle the last step of the flow which is writing to a BigQuery table.</p>
  
  <p>Not sure if I need to use windowing is what I want.  Most of the windowing examples I see are more geared towards counting over a given time period.</p>