mlr3 1.2.0

• feat: Add mirai support for parallelization and encapsulation.
• feat: Fallback can now be configured to only be used in case of certain errors via the when argument.
• feat: Custom error and warning classes.
• fix: $selected_features returns error when model is not trained yet.
• docs: Missing values during scoring.
• BREAKING CHANGE: Removed data_format argument of $data() method of DataBackend.
• BREAKING CHANGE: Remove data_formats field from Learner.
• BREAKING CHANGE: Remove DataBackendMatrix class.
• feat: Add materialize_view() method to Task to replace the internal DataBackend with a new one after operations like $select() and $filter().
• docs: Information about quantile prediction.
• perf: Use fget in assert_predictable.
• feat: Store oob error in state without requiring storing the model.
• fix: $levels() of Task returns in the correct order.
• chore: Only print up to 10 classes in the Task printer.
• fix: Check if quantiles and quantile_response are set.