Refactor / rewrite the helper functions extract_prediction & extract_prediction_list

[MalteKurz]

• Functions are quite complex for the functionality that they should implement
• Code duplication due to dependency to mlr3 version
• Use data.table functionalities in a more readable (maybe also more efficient) way

[MalteKurz]

This was done in https://github.com/DoubleML/doubleml-for-r/pull/67 and released in 0.2.0.