Implemented forceUpdate to force computation of the metrics
even if the digests haven't changed.
Moved all plotting to util.PlotMaker class with methods like:
plotMaker.plotPSD() , plotMaker.plotCoherence(), etc.
that use jfree plotting.
If plots for a particular metric are desired, just add a line like:
<cfg:argument cfg:name="makeplots">true</cfg:argument>
in between the <metric> ... </metric> tags