DMN17 — Add dedicated conversion functions for durations

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).
For consistency with the other semantic domains and make model validation more accurate I suggest adding two new conversion functions and deprecate the duration function. This will make the models more precise.