Profile the results() method of plot_set().  In conjunction with this,
I changed the plot_set3 methods to use the corresponding base class methods,
similarly to plot_set4 and plot_set5.
Finally, I sorted the file listing used in generating a name for a
cached filetable.  In reductions.py I found that the sort is necessary
to ensure consistency between runs, necessary to get re-use.
re-use the xml file generated by running cdscan.  The cdscan process
is the most expensive part of the whole calculation, at least in simple
cases.  The xml file is named with some care, to ensure that it will be
re-used only when there have been no changes in the files which it
reflects.