Add a DoubleML Framework Class

[SvenKlaassen]

Description

Add DoubleMLFramework class, which is able to concatenate or add DoubleML objects and compute the corresponding confidence intervals.
The class only operates on scaled scores $J^{-1}\psi$ and the estimates.
Aggregation of estimates, bootstrap and CI procedures are moved to the DoubleMLFramework class.
Add basic coverage tests for DoubleMLFramework

Comments

Update var_scaling_factors to a vector valued scaling for each treatment.

Open points

• Extended Exceptions with tests
• Move and test p-value adjustments
• Move Confidence Interval calculation of DoubleML and DoubleMLQTE to DoubleMLFramework

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.