- Add constructor that allow to specify separate objects for the histogram and function
      observables. This will allow the input of a RooHistFunc to be transformed.

    - Add data member _histObsList and increment schema version to v2

    - Adjust evaluate() and analyticalIntegral() to transfer contents from pdf to histo
      observables prior to evaluation

    - Implement ioStreamerPass2() to fill _histoObsList with a clone of _depList in case
      it was empty to implement correct schema evolution

  o RooAbsArg

    - Add new virtual methods canNodeBeCached() and setCacheAndTrackHints() to allow operator
      classes to steer level-2 likelihood optimization

    - Modify findConstantNodes() to skip marking any node as constant if the "NeverConstant"
      flag is set

  o RooObjCacheManager

    - Add method setClearOnRedirect() to allow modifying the 'clear on redirect' flag
      after construction

    - Bug fix in findConstantNodes() forwarder, keep in mind that cache can be sterile

  o RooProdPdf, RooProduct, RooRealSumPdf, RooAddPdf

    - Implement canNodeBeCached() to return NotAdvised and implement setCacheAndTrackHints()
      to contain the code that was so far in RooAbsObsTestStatistic (that labels all
      components as TrackAndCache)

  o RooAbsOptTestStatitic

    - Delegate level-2 likelihood optimization decisions to pdf components via the
      new interface canNodeBeCache() and setCacheAndTrackHints() and remove hard-coded
      choices here