Fix bug in a bugfix, and delete debugging printouts.
Deal with some more cases where an input argument might be None
- Make plot sets 5 and 6 just a child of a combined "plot set 5 and 6",
similar to the NCAR diagnostics where the 5 and 6 menus are combined.
- The level set case, as in the "Jerry Potter" plot set, is now entirely in
plot sets 5 & 6.  And I expanded it from model only to model, obs, and
model-obs.
With some different handling of None objects, this can now deal better
with the case where one plot of a composite plot can't be plotted.
The rest still will be plotted.