Add support for the boolean optimization in TFormula::GetExpFormula

        Prevent TTree::Scan from printing random value for invalid formulas

        Add a new TTreeFormula special function:

      Alt$(primary,alternate) : return the value of "primary" if it is available
                    for the current iteration otherwise return the value of "alternate".
                    For example, with arr1[3] and arr2[2]
      tree->Draw("arr1-Alt$(arr2,0)");
                 will draw arr[0]+arr2[0] ; arr[1]+arr2[1] and arr[1]+0
                    Or with a variable size array arr3
       tree->Draw("Alt$(arr3[0],0)+Alt$(arr3[1],0)+Alt$(arr3[2],0)");
                    will draw the sum arr3 for the index 0 to min(2,actual_size_of_arr3-1)
                    As a comparison
       tree->Draw("arr3[0]+arr3[1]+arr3[2]");
                    will draw the sum arr3 for the index 0 to 2 only if the
                    actual_size_of_arr3 is greater or equal to 3.
                    Note that the array in 'primary' is flatened/linearilized thus using
                    Alt$ with multi-dimensional arrays of different dimensions in unlikely
                    to yield the expected results.  To visualize a bit more what elements
                    would be matched by TTree::Draw, TTree::Scan can be used:
       tree->Scan("arr1:Alt$(arr2,0)");
                    will print on one line the value of arr1 and (arr2,0) that will be
                    matched by
       tree->Draw("arr1-Alt$(arr2,0)");


git-svn-id: http://root.cern.ch/svn/root/trunk@7780 27541ba8-7e3a-0410-8455-c3a389f83636