mlr3 0.13.0

• Learners which are capable of resuming/continuing (e.g.,
  learner (classif|regr|surv).xgboost with hyperparameter nrounds updated)
  can now optionally store a stack of trained learners to be used to hotstart
  their training. Note that this feature is still somewhat experimental.
  See HotstartStack and #719.
• New measures to score similarity of selected feature sets:
  sim.jaccard (Jaccard Index) and sim.phi (Phi coefficient) (#690).
• predict_newdata() now also supports DataBackend as input.
• New function install_pkgs() to install required packages. This generic works
  for all objects with a packages field as well as ResampleResult and
  BenchmarkResult (#728).
• New learner regr.debug for debugging.
• New Task method $set_levels() to control how data with factor columns
  is returned, independent of the used DataBackend.
• Measures now return NA if prerequisite are not met (#699).
  This allows to conveniently score your experiments with multiple measures
  having different requirements.
• Feature names may no longer contain the special character %.