[Feature Request]: Abstract DoubleMLBase class

[SvenKlaassen]

Describe the feature you want to propose or implement

• An abstract DoubleMLBase class, which defines certain fixed properties and methods to simplify maintainance for classes such as DoubleML, DoubleMLAPOS and DoubleMLQTE.
• Synchronize dimensions of parameters in DoubleML. Change general structure of scores to (n_obs, n_treatments, n_rep) and estimates to (n_treatments, n_rep)
• Rename parameters as coef to theta
• Improve/modularize the __str__ method by constructing general getter methods for e.g. _get_data_summary, ...

Propose a possible solution or implementation

No response

Did you consider alternatives to the proposed solution. If yes, please describe

No response

Comments, context or references

No response