Add EventView handling to AvalancheScheduler

Mostly discussed here: https://its.cern.ch/jira/browse/GAUDI-1306

Recent design presentation https://indico.cern.ch/event/632442/contributions/2718811/attachments/1526541/2387104/EventViews.pdf

Older, introductory material https://indico.cern.ch/event/578275/contributions/2342808/attachments/1416698/2169616/EventViews.pdf

Adds a method to IScheduler.h to attach EventViews to the PrecedenceRulesGraph, so that parts of the graph can be processed multiple times per event (or skipped entirely).

No measurable performance impact for non-view workloads.

May require resolution of other MRs such as !449 (closed)