tmp: Allow '.ref()' for const 'tmp'

const-ness of the stored object is checked at run-time.

src/OpenFOAM/memory/tmp/tmp.H

@@ -124,7 +124,7 @@ public:
 
             //- Return non-const reference or generate a fatal error
             //  if the object is const.
-            inline T& ref();
+            inline T& ref() const;
 
             //- Return tmp pointer for reuse.
             //  Returns a clone if the object is not a temporary

src/OpenFOAM/memory/tmp/tmpI.H

@@ -158,7 +158,7 @@ inline Foam::word Foam::tmp<T>::typeName() const
 
 
 template<class T>
-inline T& Foam::tmp<T>::ref()
+inline T& Foam::tmp<T>::ref() const
 {
     if (isTmp())
     {