ghts applied squared

<h4>Redesign of RooFit dataset class structure</h4>

The original class structure of RooFit featured an abstract dataset
class <tt>RooAbsData</tt>.  Inheriting from that was a single class
<tt>RooTreeData</tt>, which implemented datasets with a ROOT
<tt>TTree</tt>-based storage implementation, and inheriting from that
two classes <tt>RooDataSet</tt> , representing unbinned data, and
<tt>RooDataHist</tt>, representing binned data. A main problem with
this structure was that the implemen