Add uncertainty

Add the possibility to print an uncertainty in siFormat and ValueLabel. The uncertainty is printed with one significant number and the precision of the value is adjusted appropriately. Also add unittest for siFormat, siEval, siScale and ValueLabel. In addition correct a little bug found with the unittest, the character µ is not the same as μ but both should be recognized by siEval.
I think I understand the purpose of some of your formatting changes (to keep the location of +/- and units stationary?), however it is a bit unintuitive for the formatting to change due to seemingly unrelated options. It would be nice if we could work out a way for formatting to be consistent.