ENH: use exprString expansions for #eval

- follows the principle of least surprise if the expansion behaviour
  for #eval and expressions (eg, exprFixedValue) are the same.  This
  is possible now that we harness the regular stringOps::expand()
  within exprString::expand()

src/OpenFOAM/primitives/strings/stringOps/stringOps.C

@@ -252,7 +252,7 @@ static inline std::string entryToString
 
             if (pe.size() == 1 && pe[0].isStringType())
             {
-                // Already a string-type. Just copy.
+                // Already a string-type (WORD, STRING, ...). Just copy.
                 str = pe[0].stringToken();
             }
             else