GROUP
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Reference to a group of devices whose aggregate power is constrained, given as either a power sensor (``{"sensor": <id>}``) or an asset (``{"asset": <id>}``) - exactly one of the two.
The referenced sensor or asset should itself get its own flex-model entry defining the group's ``power-capacity`` (hard constraint) and/or ``consumption-capacity``/``production-capacity`` (soft constraints with default breach prices).
When the group is referenced by ``sensor``, the group's scheduled aggregate power is saved to that group sensor.
When the group is referenced by ``asset`` (e.g. a sub-EMS asset in the tree), the group entry defines no power sensor of its own; the group's aggregate power is instead saved via that entry's own ``consumption`` and/or ``production`` output sensors, following the usual output-sensor conventions.