Schedules - allow updates to single values while schedules are running
Context: how to use IEC 61850 for DER and microgrids (IEC 61850-90-23 CD1).
IEC 61850-7-4 prescribes that schedules may only be updated whilst they are offline. This is a conservative approach that allows for validation of all the settings to ensure consistency before re-starting i.e. evaluating the schedule output.
There are two approaches:
1. Stop the running schedule; change it, restart
2. Write to another schedule with a higher priority and start that; stop the original schedule; change it, restart it; stop the second schedule.
This approach is unnecessarily complex for some cases. A common use case is to update a schedule for one or more periods in the future. In these cases, updating individual schedule values will not affect the current schedule output, so there is no need to require the schedule to be deactivated and reactivated.
See also #3098