plotFeatureDensity - Plot the densities of a single feature
  as a function of class. With one input the supplied dataset
  must have only a single feature. Otherwise, the second input
  specifies which feature to plot.
 
  plotHandles = plotFeatureDensity(prtDataSetClassObj)
  plotHandles = plotFeatureDensity(prtDataSetClassObj, featureInd)
  [...] = plotFeatureDensity(prtDataSetClassObj, featureInd, 'PARMNAME', PARAMVALUE,...)
 
    Additional parameters:
        nDensitySamples        - Number of linearly spaced
                                 samples used to construct the
                                 density estimate. Default 500.
        minimumKernelBandwidth - minimumBandwidth parameter of
                                 prtRvKde that is used to
                                 estimate each density. default
                                 []. See prtRvKde.
 
  Example:
     ds = prtDataGenMary;
     plotDensity(ds,2)
 
     ds = prtDataGenIris;
     plotFeatureDensity(ds,1,'minimumKernelBandwidth',5e-3);
 
     plotFeatureDensity(ds.retainFeatures(2),'nDensitySamples',20);