Change plot_set from old-style class to a new-style class, so we can
call __subclasses__, for convenience in implementing list_diagnostic_sets.
See amwg.py for how this works.
- Change the plot_set5 implementation to the newer method, where most of the
work is done by the base class; the same as the current plot_set6.
- AMWG.list_diagnostic_sets() now computes the list, using a call of
__subclasses and a name attribute in every plot set class.  This requires
every AMWG plot set to derive from a new stub class, amwg_plot_set, and
to have a name attribute if you want it to be visible in the GUI.