3VL for expectation results

[flbulgarelli]

It would be nice that mulang could return ternary-logic value that express whether:

• the expectation was met
• the expectation was not met
• we can't decide since a precondition was not met. That happens mostly when you work with bound inspections: if the bound scope does not exist, we can not assert the expectation is met or not.

I know this is a heavy non-backward compatible change, and more a wish than a real request.