add missing import
New class level_variable_for_plot_set5, inherits from basic_level_variable.
The "variable options" method is moved from basic_level_variable to the new class.
The reason is that these auxiliary options don't just depend on the variable, they
also depend on the plot set.
Tweak naming a little.
If there are >3 plots, we don't have templates for them.  So make png files for only the first 3 plots.
Comment-out some contour setting code which is no longer necessary and in fact creates a visible bug.
Increase the number of contour levels from 10 to 16.
Use new class basic_level_variable_for_amwg_set5 for plot sets 5 & 6.