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.

Add two new functions with distinct return types

For consistency with the other semantic domains and make model validation more accurate, two new conversion functions are added and the duration function deprecate. This will make the models more precise.