DMN12 — No item definition for function definition

If a function definition is the value expression of an information item, then that information item should have an item definition that gives the type of the function definition. E.g., the type of a function of two numeric arguments that returns their sum should be something like function(number, number) returns number but we have no way to express such an item definition

needed, but out of time

Should enhance item definition so it can model function types because that is the type of a BKM's logic