BUG: dangling dictionary reference for expression BCs

- exposed by the new embedded function handling.

  Requires local copies of dictionary content instead
  (similar to coded BCs handling)

BUG: incorrect formatting for expression function output

ENH: simpler copyDict version taking wordList instead of wordRes

- corresponds to the most common use case at the moment

ENH: expression string writeEntry method

- write as verbatim for better readability

src/OpenFOAM/expressions/exprString/exprString.H

@@ -183,6 +183,19 @@ public:
 
         //- Move assign from string. No expansions, no comment stripping
         inline exprString& operator=(std::string&& str);
+
+
+    // Write
+
+        //- Dictionary entry for expression string, normally suppressing
+        //- empty strings. Generally used verbatim output (readability)
+        //  \return true if entry was written
+        bool writeEntry
+        (
+            const word& keyword,
+            Ostream& os,
+            bool writeEmpty = false
+        ) const;
 };