Fix Update_Precondition directive

SimPEG/Directives.py

@@ -305,11 +305,24 @@ class Update_IRLS(InversionDirective):
         self.reg._W = None
 
 class Update_lin_PreCond(InversionDirective):
-
-
+    """
+    Create a Jacobi preconditioner for the linear problem
+    """
+    onlyOnStart=True
+    
+    def initialize(self):
+    
+        if getattr(self.opt, 'approxHinv', None) is None:
+            # Update the pre-conditioner
+            diagA = np.sum(self.prob.G**2.,axis=0) + self.invProb.beta*(self.reg.W.T*self.reg.W).diagonal() #* (self.reg.mapping * np.ones(self.reg.curModel.size))**2.
+            PC     = Utils.sdiag(diagA**-1.)
+            self.opt.approxHinv = PC
+            
     def endIter(self):
         # Cool the threshold parameter
-
+        if self.onlyOnStart==True:
+            return
+            
         if getattr(self.opt, 'approxHinv', None) is not None:
             # Update the pre-conditioner
             diagA = np.sum(self.prob.G**2.,axis=0) + self.invProb.beta*(self.reg.W.T*self.reg.W).diagonal() #* (self.reg.mapping * np.ones(self.reg.curModel.size))**2.