This is supposed to implement the outcome of T105623: [Task] Investigate quantification of quantity precision (+/- 1 or +/- 0.5)

Namely, the "guessed" precision (the size of the uncertainty interval) should be the order of magnitude of the least significant digit. This means that 2 would be estimated to have the precision interval +/-0.5 = [1.5, 2.5) which has size 1.

Other examples:

- -0.07 should be interpreted as -0.07+/-0.005 (not +/-0.01)
- 12.6e5 should be interpreted as 1,260,000+/-5000 (not +/-10000)

The rule is that all numbers within the uncertainty interval should round to the nominal value (when rounding to the significant digits that correspond to the order of magnitude of the uncertainty interval).