Update for lat lon storage

Changed storage object of lats and lons from a tuple in the data
store to a dictionary with keys 'latitude' and 'longitude'. This is
due to the Arakawa rotate grid (32769) that will generate rotation
angles for vector quantities and we will want to store those as well.

grib2io/_grib2io.py

@@ -940,7 +940,8 @@ def grid(self, unrotate=True):
         longitudes in units of degrees.
         """
         if self._sha1_section3 in _latlon_datastore.keys():
-            return _latlon_datastore[self._sha1_section3]
+            return (_latlon_datastore[self._sha1_section3]['latitude'],
+                    _latlon_datastore[self._sha1_section3]['longitude'])
         gdtn = self.gridDefinitionTemplateNumber.value
         gdtmpl = self.gridDefinitionTemplate
         reggrid = self.gridDefinitionSection[2] == 0 # This means regular 2-d grid
@@ -1020,7 +1021,8 @@ def grid(self, unrotate=True):
         else:
             raise ValueError('Unsupported grid')
 
-        _latlon_datastore[self._sha1_section3] = (lats,lons)
+        _latlon_datastore[self._sha1_section3] = dict(latitude=lats,
+                                                      longitude=lons)
 
         return lats, lons