Explicit support for Events in the standard

[mrshll1001]

Related to #492 as it relates to schedule.

We may need to provide more explicit support (either guidance or models) for Events in the standard.

Events can be tricky to define in the context of service delivery. Sometimes a service or multiple services may be delivered at events. Sometimes an Event may have a recurring e.g. annual schedule and be related to services, but is inappropriate to model as a Service in-and-of itself.

At the moment there are reports of people declaring some events as a Service with a schedule of exactly one day.

We should discuss how events work with implementors and service providers, and determine how best to provision this as a feature in the standard. It could have either MINOR or MAJOR implications for versioning depending on implementation and should be considered alongside discussions of the schedule table.