delete obsolete function list_variables
delete obsolete script at the bottom, which originally would run
this separately from the UVCDAT GUI
New all_variables method which is like list_variables but returns a dict.
This will be used to set up a menu for any additional parameter required
to plot the variable.
New all_variables method which is like list_variables but returns a dict.
Thus we can create a variable object (basic_plot_variable class) and, if
necessary, that can tell us what additional parameters may be needed to
define a plot of the variable.
New classes basic_plot_variable and its child basic_level_variable.
The idea is that a string is not sufficient to represent a variable,
we need a class.  The reason for the class is that we can attach to
it a method which will tell us whether additional parameters need to
be defined in order to plot the variable (and what values those parameters
may take).