Fixes for JIRA-5298

  o RooAbsOptTestStatistic

    - In initSlave() move reattachement of branch buffers from interval vars to pdfs vars
      _after_ data->checkValidBins() as the latter won't work anymore after the redirect.

  o RooAbsPdf

    - Support specification of multiple fit ranges createChi2(RooDataHist&,..) and
      thus implicitly in fitChi2To(RooDataHist&...).

  o RooAbsReal

    - Redirect call of createChi2 with TList of RooCmdArgs to createChi2 with
      explicit list of arguments instead of calling RooChi2Var ctor directly in both
      (so that only one of the two needs to overridden in derived class RooAbsPdf)