a fix for SUB=TH1F::Sumw2 error in DQM/Physics package

A fix needed for issue found in here : https://github.com/cms-sw/cmssw/pull/6177
Adds a check if Sumw2 call was done before.
Before when running a workflow (4.6) where multiple runs were processed bookHistograms method was called one than once and it triggered a "Sum of squares of weights structure already created" error.