Unify convention to transform global->detPlane frame

Fixes the way that Z-axis offset of the detection plane wrt PMT panel is given. In principle this offset is used only for plotting two PMT panels side-by-side (so has no effect on PID etc), but with this fix consistency is provided. The effect of this fix will not be even visible on regular plots that could be affected by this offset, since they're made in XY plane - and the offset is only in Z.