Consolidate columns 1

Provides a library function for consolidating the senders in an activity dataframe, based on a list of tuples. And demonstrates that functionality in the iPython notebook to consolidate several senders and re-graph in order to see the difference.

Might need more documentation in the notebook.

(Not sure about why no automatic merge. Might be because this depends on changes in my previous pull request. We can ignore this one if you like and I'll continue working on it in my fork for a while.)