ENH: generalize string expression evaluation

- replace stringOps::toScalar with a more generic stringOps::evaluate
  method that handles scalars, vectors etc.

- improve #eval to handle various mathematical operations.
  Previously only handled scalars. Now produce vectors, tensors etc
  for the entries. These tokens are streamed directly into the entry.

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

@@ -570,9 +570,10 @@ static Foam::string recursiveExpand
 
                 ///Info<< "eval <" << out << ">" << endl;
 
-                // Even with allow empty, expressions need content
-                const scalar sval = stringOps::toScalar(out);
-                const word val(Foam::name(sval));
+                // Even with allow empty, expressions may need content
+
+                string val(stringOps::evaluate(out));
+                stringOps::inplaceTrim(val);
 
                 return val;
             }