Bandwidth.

    Formula:
    bw = u_bb - l_bb / m_bb

def bandwidth(data, period, std=2.0):
    """
    Bandwidth.

    Formula:
    bw = u_bb - l_bb / m_bb
    """
    catch_errors.check_for_period_error(data, period)

    period = int(period)
    bandwidth = ((upper_bollinger_band(data, period, std) -
                 lower_bollinger_band(data, period, std)) /
                 middle_bollinger_band(data, period, std)
                 )

    return bandwidth