[change] rounding model

[Mc-Zen]

This breaking change improves the design of the rounding model implemented in Zero to better conceptually mirror applications that mix values with and without uncertainties.

First and foremost, rounding of values with and without uncertainty is now separated by default. In the case that an uncertainty is present, what one usually wants is to round the value to the same digit as the uncertainty. This behavior is engaged by setting round.follow-uncertainty to true (which is the default). In this setting and optionally, the round.uncertainty-precision can be used to control the number of significant digits to round the uncertainty to before rounding the value.

If round.follow-uncertainty is false or the number just has no uncertainty, the normal rounding method applies. In this case, there is of course no guarantee that value and uncertainty have the same number of digits.

With this change the option round.mode: "uncertainty" is removed and replaced by the new parameters.