Add "Precision" to the ExpectationValue class

[AthenaCaesura]

To better implement ZNE, we would like to have access to the precision of our ExpectationValues.

So we should add a precision property to the ExpectationValues class and update calculate_exact_expectation_values and estimate_expectation_values_by_averaging in order to calculate it.

There are other functions which need to calculate precision but if they are too difficult, just leave the precision as None.

When naming your branch for the PR, make sure to put "ZQS-1132" somewhere in the name so that jira notifies us that the story has been completed.