minor changes for testing LMWG plot set 2, PREC
commented out some bulky prints
All changes are for lmwg_plot_set2.
1. reorganized the way that the derived variable names get into various places in the system, so
that they appear in both the _list_variables and _all_variables output, and are available where
needed in plan_computation.
This needs further thought and work because each name appears in at least two places, which is one
too many.
2. reorganized where reduced and derived variables get set up, to ensure: (1) derived variables
don't get into the reduced variable list, (2) when a derived variable needs a reduced
variable, it will have been defined. (3) the variableid for RAIN_x and SNOW_x is RAIN or SNOW
respectively.  Note that the variableid is used to look up the variable in a filetable, and
thus to get the data.