text: Title: add functionality to detect introduction of NAN in transform method of discretizers
  Body: For the arbitrary, equal width and equal frequency, we should include in the transform method a check to see if NaN are being introduced accidentally.
  
  This can happen if a value is outside the boundaries entered by the user in the arbitrary, or of the variable is too skewed for the other transformers.
  
  The catch should be as a warning to begin with, not to break backwards compatibility perhaps, and should inform the user exactly in which variables the NaN were introduced.
  
  We should also expand the tests of all the transformers to make sure this functionality works. As it is the same functionality for all transformers, and also for the categorical encoders as well as per issue #294, I wonder if we should make a master function from this? I think it depends in the amount of code, if it is a one liner probably not.