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/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_;