From Axel and Silvia Arcelli:

* implement rebinning, add, multiply, divide (incl. binomial)
* fix error calculation, now similar to TH1 etc
* allow for non-equal bin sizes: call constructor without bin edges,
call SetBinEdges() afterwards


git-svn-id: http://root.cern.ch/svn/root/trunk@21041 27541ba8-7e3a-0410-8455-c3a389f83636