Background work for THG's work on Cost Model

Check on the following:

1. Are aggregations supported by the DMR++ code?

   Yes, tests added to the NcML handler for this.

2. Are the URL tags we used before still supported? -

   YES, See void Chunk::add_tracking_query_param()

   and get_data_url().

3. Can the code insert a tag that's unique to a a specific request?

   Yes. Added. Uses the BES Context ‘cloudydap’. See Chunk for more info.