Enhance `cols_merge_uncert()` so that different upper and lower bounds can be used

[rich-iannone]

The cols_merge_uncert() function works nicely in cases where the uncertainty can be expressed by a single value after a plus-minus sign. However, there are cases where uncertainties differ in the positive direction and the negative direction.

To express properly this we need to extend the function to allow examination of two columns for uncertainty values (lower bound and upper bound to uncertainty), check if they are different, and use a different notation for expressing such uneven uncertainty values. This is typeset with superscripted and subscripted values for the bounds with prominent plus and minus signs. Both uncertainty values must begin at the same position (just after the measurement value) but have enough vertical space between them.