give Zero a transpose

tests/utils/test_Zero.py

@@ -1,5 +1,5 @@
 import unittest
-from SimPEG.Utils import Zero, Identity, sdiag
+from SimPEG.Utils import Zero, Identity, sdiag, mkvc
 from SimPEG import np, sp
 
 class Tests(unittest.TestCase):
@@ -29,6 +29,11 @@ class Tests(unittest.TestCase):
         assert a == 1
         self.assertRaises(ZeroDivisionError, lambda:3/z)
 
+        assert mkvc(z) == 0
+        assert sdiag(z)*a == 0
+        assert z.T == 0
+        assert z.transpose == 0
+
     def test_mat_zero(self):
         z = Zero()
         S = sdiag(np.r_[2,3])