Logging of root histograms not as precise as it could be

We now have histograms logged as counters by the MessageSvc and for each of them mean, stddev, skewness and kurtosis are logged.

In order to get these numbers, the MessageSvc has to recompute them from the content of the histogram. Thus the values printed are not computed on the original data, but on the data from the histogram. This may change slightly the results as any entry will be associated to the center of its bin rather than to its actual value.

This change had been already discovered in the RootHistogramSink some time ago and led to the development of RootHistogram, which keeps enough data to compute the mean and stddev from the data. However, we do not use these extra data in the Messagesvc. We should. Note however that we would fix only mean and stddev, we do not have enough for skewness and kurtosis