tweak DomainObject comparisons

This change should make comparisons between DomainObjects (which happen quite often) much faster.

nifty/field_types/field_array.py

@@ -24,6 +24,8 @@ class FieldArray(FieldType):
 
     def __init__(self, shape):
         super(FieldArray, self).__init__()
+        self._needed_for_hash += ["_shape"]
+
         try:
             self._shape = tuple([int(i) for i in shape])
         except TypeError: