DMN17 — duration() function has two return types

Currently, the standard supports two semantic domains for durations: years and months duration and days and time duration. However, there is one single conversion function from string that covers both semantic domains (see Table 73: Semantics of conversion functions). This makes static analysis of model difficult or impossible (in case of dynamic parameters). In B-FEEL, duration() might return the wrong type because it has to pick one of the two.