Update Variance aggregation

[SvenKlaassen]

Updating the variance aggregation over multiple repetitions.

The aggregated variance is now based on the median aggregation of the confidence intervals. The confidence interval aggregation is based on Generic Machine Learning Inference on Heterogeneous Treatment Effects in Randomized Experiments, with an Application to Immunization in India.

Aggregation is now simplified and does not require variance scaling.

PR Checklist

Please fill out this PR checklist (see our contributing guidelines for details).

• The title of the pull request summarizes the changes made.
• The PR contains a detailed description of all changes and additions.
• References to related issues or PRs are added.
• The code passes all (unit) tests.
• Enhancements or new feature are equipped with unit tests.
• The changes adhere to the PEP8 standards.

[PhilippBach]

Hi @SvenKlaassen

Thanks for opening the PR. This looks good to me. I think that really makes sense to align the standard errors and the confidence intervals in the standard summary