Changes a column of a DataFrame according to a given function
    Parameters:
    df - DataFrame
        DataFrame to operate on
    col_name - string
        Name of column to modify
    func - function
        The function to use to modify the column

def col_mod(df,col_name,func,*args,**kwargs):
    """ Changes a column of a DataFrame according to a given function
    Parameters:
    df - DataFrame
        DataFrame to operate on
    col_name - string
        Name of column to modify
    func - function
        The function to use to modify the column
    """
    backup = df[col_name].copy()
    try:
        return_val = func(df,col_name,*args,**kwargs)
        if return_val is not None:
            set_col(df,col_name,return_val)
    except:
        df[col_name] = backup