Get petsc vec (#3657)

* _ad_to_petsc/_ad_from_petsc

firedrake/adjoint_utils/function.py

@@ -398,5 +398,17 @@ def _applyBinary(self, f, y):
             npdata[i] = f(npdata[i], npdatay[i])
         vec.set_local(npdata)
 
+    def _ad_from_petsc(self, vec):
+        with self.dat.vec_wo as self_v:
+            vec.copy(result=self_v)
+
+    def _ad_to_petsc(self, vec=None):
+        with self.dat.vec_ro as self_v:
+            if vec:
+                self_v.copy(result=vec)
+            else:
+                vec = self_v.copy()
+        return vec
+
     def __deepcopy__(self, memodict={}):
         return self.copy(deepcopy=True)