mlr3tuning 0.2.0

• TuningInstance is now TuningInstanceSingleCrit. TuningInstanceMultiCrit
  is still available for multi-criteria tuning.
• Terminators are now accessible by trm() and trms() instead of term() and
  terms().
• Storing of resample results is optional now by using the
  store_resample_result flag in TuningInstanceSingleCrit and
  TuningInstanceMultiCrit
• TunerNLoptr adds non-linear optimization from the nloptr package.
• Logging is controlled by the bbotk logger now.
• Proposed points and performance values can be checked for validity by
  activating the check_values flag in TuningInstanceSingleCrit and
  TuningInstanceMultiCrit.