Method to control which weights have sumw2 calculated

[benkrikler]

It would be good to offer control over which weights have theirs squared values summed (sumw2) since in many cases in the current uses that I know of these additional columns are never looked at. And since this adds an extra column for each weight, in cases where a large number of weights are included this can be a significant memory saving and therefore I/O time, in the case of csv output format.