Add code suggested by Thomas Schietinger at https://savannah.cern.ch/bugs/?29117

"May I suggest to include a CompareArg() in TGraph, similar to CompareRadius(), i.e. to provide (just for convenience) the possibility to sort the TGraph according to angle. This is useful when points lie on an ellipse or so, but are not in order."


git-svn-id: http://root.cern.ch/svn/root/trunk@19708 27541ba8-7e3a-0410-8455-c3a389f83636