text: Title: "as_categorical"
  Body: Another idea!
  
  ```python
  @pf.register_dataframe_method
  def as_categorical(df, column_name, categories=None, ordered=True):
      """
      Convert one column in the dataframe to a categorical one.
      """
      if categories is None:
          categories = sorted(df[column_name].unique())
      df[column_name] = pd.Categorical(
          df[column_name], categories=categories, ordered=ordered,
      )
      return df
  ```
  
  It replaces repeatedly doing this piece:
  
  ```python
  df[column_name] = pd.Categorical(
      df[column_name], categories=categories, ordered=ordered,
  )
  ```
  
  Leaving this here for now.