Event.with_psc_data, a method to load scheduled activities for events

This pull request contains an implementation of `Event.with_psc_data`, a method to load scheduled activities from PSC for a set of Events.

Also contained in this PR is a change of `Event#scheduled_activities` to `Event#load_scheduled_activities` and scheduled activity caching.  This does not introduce any additional test failures, but I find it odd that `Event#scheduled_activities` was evidently not used anywhere else.  Review of this situation would be appreciated.

This addresses https://code.bioinformatics.northwestern.edu/issues/issues/show/4085.