In interpret_region, raise an exception if a region is provided as a string which cannot be found
in defines.all_regions.
In select_region, check more carefully (but maybe not carefully enough yet) whether a region is
the "global" region.  If so, do nothing.  The region subsetting expression mv(latitude=(...),...)
doesn't work well on global region.
Many changes to eliminate the former requirement that lat,lon,level axes be names 'lat','lon','lev'.
These names are checked for only as a rare last-ditch method for finding an axis.
Also there are new attributes (names of the lat,lon,level axes) in the row class, to support
changes in amwg.py which also eliminate dependence on particular axis names.
In plot set 13 (the only case where it happens), eliminate the assumption that the lat,lon axes
are named 'lat','lon'.  Instead the axis names come from the filetable - I have just changed
filetable.py to support this.
Use isLatitude() so as to not assume any particular name for the latitude axis.
Eliminate the assumption that 'lat' is the name of the latitude axis, by calling isLatitude().
However, this will run a bit quicker if 'lat' is the name of the latitude axis.
Make directories 13 and cosp if necessary.
Better checks on whether a plot exists before writting it to a png file.
Correctly check then length of a variable: len(var) won't cut it.
Entering uvc_simple_plotspec.__init__, delete None elements from pvars.  This shouldn't happen, but it
did happen.