Add the weird unit gpm from JRA25 obs data; and print a message when units can't be converted.
Put more units translations in reduce2scalar_seasonal_zonal_level - maybe not the right place,
but a quick way to fix things.
Added a file filter option to the reduced_variable class.  It's passed on to the filetable
when finding a file there.  This lets you keep track of multiple datases in the same filetable.
Fixed a typo too.
Very many changes, so that "plot set" 1, the table, works ok.  Now all numbers we print out
are either -999 (meaning that we don't compute it) or something reasonable and probably correct.
Added some more filters.  Especially notable is f_climoname.  It checks whether the file name
is typical of a climatology file.  Note however that it works only with 3-letter season
names, not with two-digit month names, e.g.
NCEP_JAN_climo.nc  or HadISST_PD_SON_climo.nc but not NCEP_01_climo.nc.
Added function parse_climo_filename and used it to identify the season that a climatology file
is for, when building the filetable.
Allow the user to supply a file filter which is used in finding files which match a variable
and its domain.  A file is returned only if the filter returns True.
Add some zonal regions used for AMWG plot set 1 (and others).
Write a table (AMWG plot set 1, at least) to a file without trying to generate plots.