Add not-in-place implementations for several dataset transforms
@lhoestq I am not sure how to test `dictionary_encode_column` (in-place version was not tested before) I can take a look at dictionary_encode_column tomorrow.
Although it's likely that it doesn't work then. It was added at the beginning of the lib and never tested nor used afaik. Now let's update the documentation to use the new methods x)
Should we deprecate in-place versions of such methods?