Reorganize a bit, and add support for doing computations on the model data before reductions.
The computations are determined by "derived unreduced variables" provided in the constructor,
and may depend on other reduced variables provided in the constructor, as well as on model
data in files.  See the comments for why these "derived unreduced variables" are treated
as second-class citizens, relative to ordinary derived variables and reduced variables.
comment out a now-annoying debugging print
Besides the climatology of a variable, average(var) (average is over time within a specified
season), compute the climatology of the variable squared, average(var^2).  This is written to a
separate file.  This takes advantage of today's change in the reduced_variable class, and is
structured so I can extend this to computation of the variable's variance (over time within a
season).
I left this script with parameters set for debugging - one season, two variables.