Change registration of discrete field for TempFields. Fix the test_fields test.

hysop/fields/tests/test_fields.py

@@ -106,7 +106,10 @@ def test_field():
     DF7.exchange_ghosts()
     DF7.accumulate_ghosts()
     exchanger = DF7.build_ghost_exchanger()
-    exchanger()
+    try:
+        exchanger()
+    except TypeError:
+        assert exchanger is None
 
     DF7.match(DF5)
     DF7 == DF5
@@ -213,7 +216,10 @@ def test_tensor_field():
     DT9.exchange_ghosts()
     DT9.accumulate_ghosts()
     exchanger = DT9.build_ghost_exchanger()
-    exchanger()
+    try:
+        exchanger()
+    except TypeError:
+        assert exchanger is None
 
     DT9.match(DT10)
     DT9 == DT10