Close-to-live curtailment no longer shows impossible values

Curtailment and compensation shown close to live previously read several times the size of the plant. A reference day is now checked for plausibility before use, and figures already issued keep their value. Plants receive this with their next agent update.

• A plant whose reference days all fail the check shows no close-to-live figure, and says so.
• After a restart of the on-site agent, a shown value no longer jumps.