changed BCM averaging range

BCM averaging window we have been using up to now does not span over the whole plateau region. On the top of that, it includes some points in the rise region, see the plot bellow.

Change proposed in this MR was already implemented in the matlab code (/clear/MatLab/Operation/MTVAcquisition/Califes_control_functions/Read_BCM.m), but was not synchronised with the rest of the BCM charge evaluating code.