ENH: ensure written and evaluated values correspond (issue #311)

- Avoids possible discrepancy when the user selects non-coincidental
  values for executeInterval and writeInterval.

- zeroGradient and ddt2 function objects

src/functionObjects/field/ddt2/ddt2.C

@@ -112,6 +112,7 @@ Foam::functionObjects::ddt2::ddt2
 )
 :
     fvMeshFunctionObject(name, runTime, dict),
+    prevTimeIndex_(-1),
     selectFields_(),
     resultName_(word::null),
     blacklist_(),
@@ -217,12 +218,20 @@ bool Foam::functionObjects::ddt2::execute()
             << "Unprocessed field " << ignored << endl;
     }
 
+    // Update time index
+    prevTimeIndex_ = obr_.time().timeIndex();
+
     return true;
 }
 
 
 bool Foam::functionObjects::ddt2::write()
 {
+    if (prevTimeIndex_ < obr_.time().timeIndex())
+    {
+        // Ensure written results reflect the current state
+        execute();
+    }
     if (results_.size())
     {
         Log << type() << ' ' << name() << " write:" << endl;

src/functionObjects/field/ddt2/ddt2.H

@@ -101,6 +101,9 @@ class ddt2
 {
     // Private data
 
+        //- Time at last execute, ensures write uses up-to-date values
+        label prevTimeIndex_;
+
         //- Name of fields to process
         wordReList selectFields_;
 

src/functionObjects/field/zeroGradient/zeroGradient.C

@@ -100,6 +100,7 @@ Foam::functionObjects::zeroGradient::zeroGradient
 )
 :
     fvMeshFunctionObject(name, runTime, dict),
+    prevTimeIndex_(-1),
     selectFields_(),
     resultName_(string::null),
     results_()
@@ -174,12 +175,20 @@ bool Foam::functionObjects::zeroGradient::execute()
             << "Unprocessed field " << ignored << endl;
     }
 
+    // Update time index
+    prevTimeIndex_ = obr_.time().timeIndex();
+
     return true;
 }
 
 
 bool Foam::functionObjects::zeroGradient::write()
 {
+    if (prevTimeIndex_ < obr_.time().timeIndex())
+    {
+        // Ensure written results reflect the current state
+        execute();
+    }
     if (results_.size())
     {
         Log << type() << ' ' << name() << " write:" << endl;

src/functionObjects/field/zeroGradient/zeroGradient.H

@@ -95,6 +95,9 @@ class zeroGradient
 {
     // Private data
 
+        //- Time at last execute, ensures write uses up-to-date values
+        label prevTimeIndex_;
+
         //- Name of fields to process
         wordReList selectFields_;