Add Simulations for APO Causal Contrast for ATE & ATTEs vs IRM results

[PhilippBach]

We could add an additional set of results to validate the coverage of APO causal contrast with appropriate re-weighting for the ATE and ATTE estimators in a setting comparable to IRM ATE and ATTE results. Results should be added here https://docs.doubleml.org/doubleml-coverage/irm/apo.html#causal-contrast-coverage

[PhilippBach]

We test the numerical equivalence of the IRM ATE & ATTE vs. the IRM APOs with appropriate weights in https://github.com/DoubleML/doubleml-for-py/blob/main/doubleml/irm/tests/test_irm_vs_apos.py

This is why I think it's enough to provide simulations for the ATE to the coverage repo...