Define semantic meaning for maxNumberOfDiagnoses <= 0 for all diagnosis engines

To the opposite of HS-Tree and HS-DAG where the default value of 0 (and all negative ints) for the member maxNumberOfDiagnoses has the semantic meaning of searching for all possible diagnoses, in Inv-HS-Tree this default value has the effect that no diagnosis is searched at all if not set explicitely by an external API call through setMaxNumberOfDiagnoses(int).

Change the semantics for 0 (and all negative ints) max diagnoses in Inv-HS-Tree to the same as it is in HS-Tree and HS-DAG.

This bug does not effect the plugin but the usage of our API. Especially if the max number of diagnoses is not set explicitely with a positive integer.