possible "syntax checks" for `FindAuxiliaryPetriNodes`

[jvoigtlaender]

• none of the counts given is negative
• the counts of auxiliary nodes is not greater than the sum of the non-auxiliary node counts (is that a variant that really always holds?)
• none of the counts is greater than the number of activity diagram elements on the screen (is that something that can be made precise, and is safe to say?)