Move most AMWG-specific code, mainly the plot_setN classes, to amwg/amwg.py.
In class BasicDiagnosticGroup, add a method list_diagnostic_sets().
This can be used to construct menus.  So every class inheriting from
this class, e.g. AMWG, LMWG, should implement this new method.
- Move AMWG-specific code, mainly plot_setN classes, here from
frontend/uvcdat.py.
- Add stubs of more plot_setN classes, yet to be defined.
- Implement a list_diagnostic_sets() method of the AMWG class, for menu
construction.