Update Sensitivity Operations & APO Model

[SvenKlaassen]

Update the sensitivity calculations in the DoubleMLFramework:

• Remove psi_sigma2 and psi_nu2 from sensitivity_elements (still contained in DoubleML)
• Add max_bias and psi_max_bias to sensitivity_elements
• Update operations +, - and *

Update APO and IRM model

• Improve DoubleMLAPO nuisance estimation for main regression (S learner for $d\neq \text{treatment level}$)
• Includes sharper bounds for causal_contrast() sensitivity analysis
• Update IRM and APO model scores: $\psi_a = -1 \cdot \omega(Y, D, X) / E[\omega(Y, D, X)] $
• Update propensity score normalization for sensitivity analysis

Further changes

• add pytest-coverage to development dependencies
• add .coverage to .gitignore

PR Checklist

• 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.