Correct zdef dimension (JCSDA-internal#185)

Change check value on `zdef `dimension. This is needed for observation
data, particularly ps obs which have only a single zdef (level) value.

This change has been tested with Ozn, Rad, and Min data and all work
correctly.

src/eva/data/mon_data_space.py

@@ -883,7 +883,7 @@ def get_dim_ranges(self, coords, dims, channo, datatypes):
         if dims['ydef'] > 0:
             y_range = channo if coords['ydef'] == 'Channel' else np.arange(1, dims['ydef']+1)
 
-        if dims['zdef'] > 1:
+        if dims['zdef'] > 0:
             z_range = np.arange(1, dims['zdef']+1)
 
         return x_range, y_range, z_range