Histograming/Sink/Utils.h: Fix calculation of stats for (profile) histograms

Closes lhcb/Gaudi#5

Fixes a number of issues in the calculations of histogram stats :-

1. Fix bin values used for profile histograms, that was not taking the correct (weighted) bin values into account.
2. Fix a bug in the calculation of the bin central value for profiles, that had an offset of one bin.
3. Also adds protection against sqrt(-ve) when values are expected to be zero, but due to machine (im)precision could fluctuate below zero.