Schedule should require either startDate or endDate

[nickevansuk]

To ensure that Schedule is used correctly, it should include at least either a startDate or an endDate: ideally both. Otherwise PartialSchedule should be used.

This ensures Schedules are always used for cases where they are based a legitimate underlying schedule.

[ldodds]

We've discussed some of this previously.

• endDate is recommended because they're not always known. Not knowing that doesn't mean we have a PartialSchedule, its just not something that is currently known. Adding an artificial endDate doesn't seem correct.
• I can see the argument for startDate. It will obviously need to be required if its included in the idTemplate and urlTemplate properties. We could validate for that. But as its not strictly required to generate events in the schedule outside of that, I'm not sure it needs to be required other than providing extra helpful information.

Tightening up conformance to ensure that variables included in the templates is something that could be added without breaking additional data. But making these properties mandatory is a breaking change.