Accept names instead of constants as input for StandardUncertainty

[c3d]

The description of #1574 contains:

I understand that this comes from the fact that in v0.9.14 these command are not or only partly implemented but I think that they should be equivalent to (or interpreted as):

1. for 'StdUnc' <=> append 'Ⓢ' before the constant and then execute →Num,
2. for 'RelUnc' <=> append 'Ⓡ' before the constant and then execute →Num.

I believe that the intent is to accept a name as input to StandardUncertainty