Combine RelationalOperator with NumericalMeasure

[ilkkarinne]

Relational operators (above or below the given value) are used in several places in the message classes as separate properties. Combining them as an Optional relational operator within the NumericalMeasure would be their use clearer since the entire value would them be accessible from a the single property, and the using code would not so easily forget to check the possible additional relational operator.