Gains averaging in Gainplots.py needs to be 64 bit

[KshitijT]

The code below (in Gainplots.py) :

        amp = abs(gg)
        ampwh = numpy.ma.masked_array(amp,mask=~valid);
        amid  = ampwh.mean(1);

needs to be changed to

        amp = abs(gg)
        ampwh = numpy.ma.masked_array(amp,mask=~valid);
        amid  = ampwh.mean(1,dtype="float64");

to avoid errors in computing the mean (amp is float32) - the current code may produce really weird plots.