text: Title: When ingest dataframe, use alternative tagging
  Body: In addition to ticket #79 
  
  Would it be possible to next to, data_frame_tag_columns=tag_columns, also have a 'data_frame_tag=' argument? This way a tag can be added to a DF which doesn't appear in the DF.
  
  For example, I have a DF with stock prices: timestamp, open, high, low, close (etc) data. I would like to be able to add tags as ticker, exchange etc which don't appear in the DF, by using a 'data_frame_tag=' argument with data_frame_tag='NASDAQ', 'AAPL'