Werk #15638: Predictive Levels: Supress levels for constant values

This change affects users of the option In relation to standard deviation from Predictive Levels.

Predictive Levels are computed by considering every data point matching the configured reference period. If the data never changes across these points in time, then the standard deviation is zero. Thus, the predicted levels will precisely equal to the predicted value. Previously, this would result in a CRIT. This happened no matter the levels set via In relation to standard deviation.

While the described behaviour is self-consistent, most of these alarms are undesired and diminish the usefulness of Predictive Levels.

With this Werk, no levels are applied, if the standard deviation is zero.

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