^^^^^^^^^^^^^^^^^^^^^^
- Updated to use basic bootstrap for confidence intervals (Fixes issue 5)
- Add PySpark UDFs for operating on histogram columns to facilitate per-user
aggregation: histogram sum, count of events, quantiles, and fraction of
events exceeding a threshold (Fixes issue 10)
- Revise metric definitions to avoid crashing in bootstrapping if a client had
NULL values after daily aggregation
- Fix `EngagementIntensity` to correctly calculated active hours / total hours