New function, to convert season from a string to a cdutil.times.Seasons object.
New function reduce2scalar_seasonal_zonal_level - same as reduce2scalar_seasonal_zonal
but restricts data (if 3D) to the specified level (very approximately).
moved amwg_plot_set4 methods reduced_variables_press_lev and reduced_variables_hybrid_lev
to independent functions, in amwg1.py, so that they may be shared by plot sets 1 and 4 and
any other classes which do the same thing.
making progress on correctly computing level set data for table entries.  It's not computed yet,
so -999.00 is returned.
Merge branch 'devel' of github.com:UV-CDAT/uvcmetrics into devel