added an __iadd__, __isub__ to zero class

SimPEG/Utils/matutils.py

@@ -399,8 +399,10 @@ def diagEst(matFun, n, k=None, approach='Probing'):
 class Zero(object):
     def __add__(self, v):return v
     def __radd__(self, v):return v
+    def __iadd__(self, v):return v
     def __sub__(self, v):return -v
     def __rsub__(self, v):return v
+    def __isub__(self, v):return v
     def __mul__(self, v):return self
     def __rmul__(self, v):return self
     def __div__(self, v): return self

tests/utils/test_Zero.py

@@ -20,6 +20,13 @@ class Tests(unittest.TestCase):
         assert 3*z == 0
         assert z*3 == 0
         assert z/3 == 0
+
+        a = 1
+        a += z
+        assert a == 1
+        a = 1
+        a += z
+        assert a == 1
         self.assertRaises(ZeroDivisionError, lambda:3/z)
 
     def test_mat_zero(self):