Middle Bollinger Band.

    Formula:
    m_bb = sma()

def middle_bollinger_band(data, period, std=2.0):
    """
    Middle Bollinger Band.

    Formula:
    m_bb = sma()
    """
    catch_errors.check_for_period_error(data, period)

    period = int(period)
    mid_bb = sma(data, period)

    return mid_bb