CI calculation based on std errors from bootstraps

[irhete]

The current implementation estimates the confidence interval directly from bootstraps. Another option would be to use bootstrapped values to get the standard error and then derive the CI from normal (or student's T) critical values. The former has worse statistical properties than the latter (you need more iterations for it to converge), but the latter expects the distribution of the estimator to be symmetric.

We could implement the std error based solution, set it as default, but keep an option for the user to switch back to the direct CI estimation.

Thanks @ilirmaci for pointing it out!