Zero and Identity - Useful for writing derivatives.

These should work with sparse matrices and numpy arrays.

```python
	z = Zero()
	z*A == 0
	o = Identity()
	o*A == A
```

SimPEG/Utils/matutils.py

@@ -396,4 +396,60 @@ def diagEst(matFun, n, k=None, approach='Probing'):
 
     return d
 
+class Zero(object):
+    def __add__(self, v):return v
+    def __radd__(self, v):return v
+    def __sub__(self, v):return -v
+    def __rsub__(self, v):return v
+    def __mul__(self, v):return self
+    def __rmul__(self, v):return self
+    def __div__(self, v): return self
+    def __truediv__(self, v): return self
+    def __rdiv__(self, v): raise ZeroDivisionError('Cannot divide by zero.')
+    def __pos__(self):return self
+    def __neg__(self):return self
+    def __lt__(self, v):return 0 < v
+    def __le__(self, v):return 0 <= v
+    def __eq__(self, v):return v == 0
+    def __ne__(self, v):return not (0 == v)
+    def __ge__(self, v):return 0 >= v
+    def __gt__(self, v):return 0 > v
+
+class Identity(object):
+    _positive = True
+    def __init__(self, positive=True):
+        self._positive = positive is True
+
+    def __pos__(self):return self
+    def __neg__(self):return Identity(not self._positive)
+
+    def __add__(self, v):
+        if sp.issparse(v):
+            return v + speye(v.shape[0]) if self._positive else v - speye(v.shape[0])
+        return v + 1 if self._positive else v - 1
+    def __radd__(self, v):
+        return self.__add__(v)
+
+    def __sub__(self, v): return self+-v
+    def __rsub__(self, v):return -self+v
+
+    def __mul__(self, v): return v if self._positive else -v
+    def __rmul__(self, v):return v if self._positive else -v
+
+    def __div__(self, v):
+        if sp.issparse(v): raise NotImplementedError('Sparse arrays not divisibile.')
+        return 1/v if self._positive else -1/v
+    def __truediv__(self, v):
+        if sp.issparse(v): raise NotImplementedError('Sparse arrays not divisibile.')
+        return 1.0/v if self._positive else -1.0/v
+    def __rdiv__(self, v):
+        return v if self._positive else -v
+
+    def __lt__(self, v):return 1 <  v if self._positive else -1 <  v
+    def __le__(self, v):return 1 <= v if self._positive else -1 <= v
+    def __eq__(self, v):return v == 1 if self._positive else v == -1
+    def __ne__(self, v):return (not (1 == v))if self._positive else (not (-1 == v))
+    def __ge__(self, v):return 1 >= v if self._positive else -1 >= v
+    def __gt__(self, v):return 1 >  v if self._positive else -1 >  v
+