[Minor] Add function to retrieve regressor coefficients

[weberpals]

🔬 Background

• Make regressor coefficients accessible via a function Unable to extract LR coefficients #1565

🔮 Key changes

• Added a new function to TimeNet Class which helps the user retrieve the coefficients for future regressors, lagged regressors, and events

📋 Review Checklist

• I have performed a self-review of my own code.
• I have commented my code, added docstrings and data types to function definitions.
• I have added pytests to check whether my feature / fix works.

Please make sure to follow our best practices in the Contributing guidelines.

[ourownstory]

Thank you Constantin!

It is looking good overall!

You are headed in the right direction - I added some comments on how you can address some issues regarding the coefficient computation.

Please also add one or more tests covering your new functionality, a micro example in the docstring, and add a short feature-guide notebook demonstrating the use of your method.

[ourownstory]

Thank you for upgrading the coefficient computation!

I left few small comments in the review. Beyond, Do you mind adding:

• Please also add one or more tests covering your new functionality,
• a micro example in the docstring,
• and add a short feature-guide notebook demonstrating the use of your method (both directly, and indirectly via the plotting methods).
• Please also look into the existing plotting code and make sure that that code uses your code for visualizing coefficients in plots. Please also make sure that if your code yields different results (compared to the existing plotting code; likely), that those differences are intentional/meaningful (also likely).

[weberpals]

Thanks for the feedback! I wasn't aware that some of the weight calculations were already available and used in the plotting function. I adapted the functions to integrate cleaner with the existing code.
The existing plot function didn't seem to work with NN future regressors, so I am using get_future_regressor_coefficients() there now. For the other coefficient calculations I am also using some of the existing code to avoid duplicate functionality.