Show asymmetric uncertainty as symmetric when upper and lower uncertainties are the same

[kh296]

If upper and lower values for an asymmetric uncertainty are the same, they're currently shown as superscript and subscript:

\num{20.01(2:2)} gives approximately the same as $20.01^{+0.02}{-0.02}$

There could perhaps be a package option to retain the current behaviour, or to recognise automatically when the two uncertainties are the same, and show them as a symmetric uncertainty:

\num{20.01(2:2)} should give approximately the same as $20.01 \pm 0.02$

This could be useful, for example, when autogenerating tables of values and their uncertainties, as the user could use the syntax for asymmetric uncertainties in all cases, without needing to check whether lower and upper uncertainties are the same or different.