Get the CPO used to create a CPOTrained object. The
retrieved CPO will usually have all its hyperparameters and affect.*
settings set to the values used to create the particular CPOTrained object.
The only case where this is not true is if cpo is a CPOTrained
that was created using makeCPOTrainedFromState.
getCPOTrainedCPO(cpo)[CPOTrained]
The Retrafo or Inverter to get the original CPO from.
Other getters and setters: CPO,
  getCPOAffect, getCPOClass,
  getCPOConstructor, getCPOId,
  getCPOName,
  getCPOOperatingType,
  getCPOPredictType,
  getCPOProperties,
  getCPOTrainedCapability,
  setCPOId
Other retrafo related: CPOTrained,
  NULLCPO, %>>%,
  applyCPO, as.list.CPO,
  clearRI, getCPOClass,
  getCPOName,
  getCPOOperatingType,
  getCPOPredictType,
  getCPOProperties,
  getCPOTrainedCapability,
  getCPOTrainedState,
  is.retrafo,
  makeCPOTrainedFromState,
  pipeCPO, print.CPOConstructor
Other inverter related: CPOTrained,
  NULLCPO, %>>%,
  applyCPO, as.list.CPO,
  clearRI, getCPOClass,
  getCPOName,
  getCPOOperatingType,
  getCPOPredictType,
  getCPOProperties,
  getCPOTrainedCapability,
  getCPOTrainedState,
  is.inverter,
  makeCPOTrainedFromState,
  pipeCPO, print.CPOConstructor
Other CPO lifecycle related: CPOConstructor,
  CPOLearner, CPOTrained,
  CPO, NULLCPO,
  %>>%, attachCPO,
  composeCPO, getCPOClass,
  getCPOConstructor,
  identicalCPO, makeCPO