A key to control download of data

Currently the query downloads all the data (inputs and outputs) besides metadata. In the WFQuery class add a keyword argument tell the query to add the actual data or not. Something like metadata_only = True or download_data = True would be sufficient.

This is if the query returns a lot of documents that can saturate the capacity of connection or exceed the memory. If we do not need to process the data (and many queries have this intention) the download is not necessary.