Allow zero as one of the values for an asymmetric uncertainty

[kh296]

In some cases, either the upper or lower value of an asymmetric uncertainty may be zero. For example, if the ratio of successes to trials for a binomial process is high, then the best estimate of the ratio from a given set of trials may be 1, in which case the upper uncertainty is 0.

The present behaviour in siunitx is:

\num{20.01(0:2)} gives the same as \num{20.01(2)} 

\num{20.01(2:0)} gives the same as \num{20.01} 

It would be nice instead if the 0 could be retained as subscript or superscript:

\num{20.01(0:2)} should give something like $20.01^{+0.00}_{-0.02}$

\num{20.01(2:0)} should give something like $20.01^{+0.02}_{-0.00}$

Thanks!