Change any columns of strings in a panda's dataframe to a column of
    categorical values. This applies the changes inplace.
    Parameters:
    -----------
    df: A pandas dataframe. Any columns of strings will be changed to
        categorical values.
    Examples:
    ---------
    >>> df = pd.DataFrame({'col1' : [1, 2, 3], 'col2' : ['a', 'b', 'a']})
    >>> df
       col1 col2
    0     1    a
    1     2    b
    2     3    a
    note the type of col2 is string
    >>> train_cats(df)
    >>> df
       col1 col2
    0     1    a
    1     2    b
    2     3    a
    now the type of col2 is category

def train_cats(df):
    """Change any columns of strings in a panda's dataframe to a column of
    categorical values. This applies the changes inplace.
    Parameters:
    -----------
    df: A pandas dataframe. Any columns of strings will be changed to
        categorical values.
    Examples:
    ---------
    >>> df = pd.DataFrame({'col1' : [1, 2, 3], 'col2' : ['a', 'b', 'a']})
    >>> df
       col1 col2
    0     1    a
    1     2    b
    2     3    a
    note the type of col2 is string
    >>> train_cats(df)
    >>> df
       col1 col2
    0     1    a
    1     2    b
    2     3    a
    now the type of col2 is category
    """
    for n,c in df.items():
        if is_string_dtype(c): df[n] = c.astype('category').cat.as_ordered()