plotPairs() - Plot each pair of features in feature space
    along the diagonals the densities of the two features are
    plotted. A DataSet.nFeatures x DataSet.nFeatures array of
    axes are created for the plots.
 
  [plotHandles, axesHandles] = plotPairs(prtDataSetClassObj)
  [...] = plotPairs(prtDataSetClassObj, containingHandle);
        containingHandle should be a valid parent for a MATLAB
        axes object such as a uipanel or a figure. The default
        is gcf.