ENH: enable 'status=done' when finishing an externalCoupled FO

- This provides a mechanism for the external code to detect when
  OpenFOAM is done.

- Adjust tutorial to use the mechanism. Also test in parallel.

src/postProcessing/functionObjects/jobControl/externalCoupled/externalCoupledFunctionObject.H

@@ -82,6 +82,7 @@ Description
         log             yes;
         commsDir        "${FOAM_CASE}/comms";
         initByExternal  yes;
+        stateEnd        remove;  // (remove | done)
 
         regions
         {
@@ -122,9 +123,11 @@ SourceFiles
 #include "DynamicList.H"
 #include "wordReList.H"
 #include "scalarField.H"
+#include "NamedEnum.H"
 #include "Switch.H"
 #include "UPtrList.H"
 
+
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
 namespace Foam
@@ -145,6 +148,23 @@ class externalCoupledFunctionObject
 :
     public functionObject
 {
+public:
+
+    // Public data types
+
+        //- Lockfile state on termination
+        enum stateEnd
+        {
+            REMOVE,     //!< Remove lock file on end
+            DONE        //!< Lock file contains status=done on end
+        };
+
+private:
+
+        //- State end names
+        static const NamedEnum<stateEnd, 2> stateEndNames_;
+
+
     // Private data
 
         //- Reference to the time database
@@ -168,6 +188,9 @@ class externalCoupledFunctionObject
         //- Flag to indicate values are initialised by external application
         bool initByExternal_;
 
+        //- Lockfile state on termination
+        stateEnd stateEnd_;
+
         //- Log flag
         bool log_;
 
@@ -209,11 +232,15 @@ class externalCoupledFunctionObject
         //- Return the file path to the lock file
         fileName lockFile() const;
 
-        //- Create lock file
-        void createLockFile() const;
 
-        //- Remove lock file
-        void removeLockFile() const;
+        //- Create lock file to indicate that OpenFOAM is in charge
+        void useMaster() const;
+
+        //- Remove lock file to indicate that the external program is in charge
+        void useSlave() const;
+
+        //- Remove lock file or status=done in lock.
+        void cleanup() const;
 
         //- Remove files written by OpenFOAM
         void removeWriteFiles() const;
@@ -221,8 +248,9 @@ class externalCoupledFunctionObject
         //- Remove files written by external code
         void removeReadFiles() const;
 
-        //- Wait for response from external source
-        void wait() const;
+        //- Wait for indication that the external program has supplied input
+        //  (ie, for the lock file to reappear).
+        void waitForSlave() const;
 
 
         //- Read data for a single region, single field