minor improvements - make None mean default season, and somewhat
better plot title
Don't crash on a variable without levels; just return None.
name change levels to level_src because it often isn't an array of levels,
it can be something else used as a source of levels
New, temporary, plot set inspired by Jerry Potter.  It plots geopotential
height, which is a special case of a level surface.  After some more
work with menus and parameterizing things, this type of plot will
become part of Plot Set 5.  In NCAR's Plot Set 5, about half the plots
are level surfaces and we don't do that yet.
New function select_lev.  This eliminates the level dimension by
selecting the value at one particular level.