text: Title: add inverse_transform method to variable transformer classes
  Body: Add to the log, power and reciprocal transformers (https://github.com/solegalli/feature_engine/tree/master/feature_engine/transformation) an inverse_transform method that performs the inverse operation. 
  
  So for log it would be exp, for exp it would be log, for 1/x it would be 1/x
  
  BoxCox and Yeojohnson, would continue without the inverse_transform. Not sure these operations can be reversed.