Handle missing station data file rather than exiting with an exception (

#198)

Add zeroed data if a station data file is missing to ensure continued
execution.

Resolves #197 

## Dependencies
Not a huge deal but I started this branch from my fix for #196 so that
should be merged before this change.

---------

Co-authored-by: Cory Martin <cory.r.martin@noaa.gov>

src/eva/data/mon_data_space.py

@@ -799,6 +799,10 @@ def read_stn_ieee(self, file_name, coords, dims, ndims_used, dims_arr,
             dims['ydef'] = numobs
             f.close()
 
+        else:
+            rtn_array = np.zeros((len(vars), 1, 1), float)
+            dims['ydef'] = 1
+
         rtn_lat = np.asarray(lat).reshape(-1)
         rtn_lon = np.asarray(lon).reshape(-1)