calculate magnetic field strength from raw magnetometer (dataflash version)

def mag_field_df(MAG, ofs=None):
    '''calculate magnetic field strength from raw magnetometer (dataflash version)'''
    mag = Vector3(MAG.MagX, MAG.MagY, MAG.MagZ)
    offsets = Vector3(MAG.OfsX, MAG.OfsY, MAG.OfsZ)
    if ofs is not None:
        mag = (mag - offsets) + Vector3(ofs[0], ofs[1], ofs[2])
    return mag.length()