mlr3 1.3.0

• feat: Learner$predict() can now add additional data to PredictionClassif and PredictionRegr objects via the extra field.
• feat: Mlr3Error and Mlr3Warning classes for errors and warnings.
• refactor: $obs_loss methods in Measure, Prediction, ResampleResult, and BenchmarkResult.
• perf: Use more lengths() and data.table::setattr() in tests.
• perf: Use attr() instead of attributes() for extracting single attributes.
• test: Use more specialised test functions.
• fix: weights_measure now work with stratum.
• fix: Encapsulation loads loaded packages on the workers.
• fix: Learners can handle new factor levels.
• refactor: Use more implicit returns.
• feat: Learner can pass extra data to predictions.