mlr3mbo 0.2.1

• docs: updated some references in vignette.
• refactor: minor clean up of the internal structure of all loop functions.
• perf: default initial design constructed based on a Sobol sequence in all loop functions.
• refactor: no longer depend on mlr3tuning but import instead.
• refactor: srlrn sugar function now can construct both a SurrogateLearner and
  SurrogateLearnerCollection; dropped srlrnc.
• feat: added AcqFunctionSD, AcqFunctionEHVI and AcqFunctionEHVIGH, introduced
  bayesopt_emo loop function.
• feat: AcqFunctions now include a $packages field stating required packages which are checked
  for whether their namespace can be loaded prior to optimization.
• fix: fixed bug in fix_xdt_missing() helper function.
• BREAKING CHANGE: renaming default_loopfun -> default_loop_function,
  default_acqfun -> default_acqfunction,
  default_acqopt -> default_acqoptimizer.
• BREAKING CHANGE: result_functions now replaced by ResultAssigners.
• BREAKING CHANGE: renamed $model field of all Surrogate classes to $learner.
• BREAKING CHANGE: For all Surrogate and AcquisitionFunction classes fields *_cols renamed to
  cols_* (e.g., x_cols to cols_x).