[Py] Complex observables don't report complex error bar

[bkesk]

As seen in the code

https://github.com/mmorale3/qmcpack2ccq/blob/1acdd1977877fc31454b5294d7df9ebfdc5c7d31/utils/afqmctools/analysis/average.py#L358-L360

The error bar reported is only for the real part, which we should also include the imaginary error