[Feature Request]: Abstract DoubleMLBase class

### 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_