Add support for binary outcome in PLR

[SvenKlaassen]

Enable the save use of classifiers for the PLR model (see #234)
The model should accept classifiers and switch to predict_proba() (but only for ml_l).

[ricor07]

Hello, I'd like to join the project and work on this.

[SvenKlaassen]

Hi @ricor07,

that would be great.
I think this should mainly require changes in the https://github.com/DoubleML/doubleml-for-py/blob/main/doubleml/plm/plr.py file.
If you have a first version you can open a PR (or if you have questions; just tag me)

Best,
Sven