Improvements to InterestPeriodDefinition class

Once event data is sent through an interest period filter, a depth-of-three MultiIndex (experiment_id, session_id, ip_id_num) is very helpful for merging. I first made the ip_id_num column enumerated in (experiment_id, session_id) chunks rather than just enumerating across all experiments and sessions. Once this is done, it makes it very easy to merge the filtered data with the ip_df in order to carry over columns like the ip start_time and end_time.

I also added a bit to the examples to compare performance on ip.filter to ip.find and demonstrate how to retain the various columns.