SessionSeries requirements for eventSchedule and subEvent should be forgiving

[nickevansuk]

SessionSeries requirements for eventSchdule and subEvent should be forgiving for split-feed implementations, as currently if the validator validates only one half of the split it is not possible to determine whether the subEvent may be defined in the other half

[lukehesluke]

Second this! Worth noting that Test Suite has a workaround for this exact reason (here), which is not ideal at all. The BookingSystem.AspNetCore reference implementation within OpenActive.Server.NET outputs SessionSeries which fail this rule (but shouldn't as it has split feeds). We at imin also have a workaround in our code for this

I think that, until this is fixed, more workarounds are going to show up, adding tech debt to the general ecosystem. It's also the recommended approach (in the dev docs) to use split feeds, so this rule is not necessarily very useful

It's not obvious how to solve this so shall we start iterating on a possible solution, @nickevansuk ? As some starter-for-10 proposals:

1. a Validator mode could be used to indicate that the SessionSeries belongs to a split feed
2. Downgrade the failure to a warning
   • this may be too forgiving, but it's worth noting that, since combined feeds are no longer recommended, this rule essentially becomes an advisory that EventSchedules be used as much as possible
3. following on from the previous point, removing this rule and instead making two extra rules:
   1. Warning: If a SessionSeries has ScheduledSessions as split feeds are recommended
   2. Warning: If a SessionSeries has no EventSchedule as this field is recommended

To clarify for any future searches, we're specifically talking about the SessionCourseHasSubeventOrScheduleRule rule