Use also absolute tolerance in TF1::Integral. This fixes the failure in stressMathCore observed after changing GaussIntegrator to use both relative and abs tolerance
(fix http://root.cern.ch/viewvc?view=rev&revision=47641 in master and merged in patches in git
 http://root.cern.ch/gitweb?p=root.git;a=commit;h=a89dd65d43eb326ebdc77b87f7fc969f1c057423 )