New file - dictionaries and functions for handling units.  udunits doesn't always cut it,
especially with the odd choices of unit names found in some obs datasets.  As time goes by,
this will have to be expanded on an ad-hoc basis.
Change reduce2scalar_zonal() and reduce2scalar_seasonal_zonal() to take better advantage of cdms2
capabilities for restricting a variable to a latitude range.  Not only is this briefer, but it is
more robust; in particular it handles missing data properly.  Thanks to Brian Smith for passing on
Charles Doutriaux's explanation of how to do this.

Also I deleted some obsolete functions and moved one almost-obsolete function, pressures_in_mb(),
to the new file units.py.
Declare expected "standard" units for a few table rows (ideally we'd do this for them all) and
make sure that output values are in these units.