Read a CSV in canonical form: ::

        <agent_id, t_step, key, value, value_type>

def read_csv(filename, keys=None, convert_types=False, **kwargs):
    '''
    Read a CSV in canonical form: ::

        <agent_id, t_step, key, value, value_type>

    '''
    df = pd.read_csv(filename)
    if convert_types:
        df = convert_types_slow(df)
    if keys:
        df = df[df['key'].isin(keys)]
    df = process_one(df)
    return df