This patch introduces a new special function in TTreeFormula Sum$
described as follow:

//  Sum$(formula): return the sum of the value of the elements of the
//  formula given  as a parameter.  For example the mean for all the
//  elements in  one entry can be calculated with:
//                Sum$(formula)/Length$(formula)

The idea comes from http://root.cern.ch/phpBB2/viewtopic.php?p=6943


git-svn-id: http://root.cern.ch/svn/root/trunk@11629 27541ba8-7e3a-0410-8455-c3a389f83636