PyAnalysisUtils: Work around cling issue with detached debug info

When TH1::Draw is called, root tries to call a TCanvas method
to create the default canvas.  If that method has not been linked,
then root will search all DSOs on the library path until it find
that method.  (This mechanism is separate from the rootmap mechanism.)
However, root will also pick up detached debug files during this scan,
but will crash trying to read them.

Work around by ensuring thad TPad has been loaded before calling Draw.