All reduce2scalar* functions allow the caller to provide weights for averaging along the
latitude axis.  This is because many CAM files specify the weights.  Actually, they are
almost always exactly the same as the default computed by the averager, but it's more
correct to use the ones specified by the data file.
It the data file has a variable 'gw', this is normally Gaussian weights for averaging in the
latitude axis.  This variable is now read in and passed on to reduce2scaler* functions so
that it will be used as intended.
In std2var and plot set 5, do better in the case filetable==None.