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.

tutorials/IO/dictionary/good-if2.dict

@@ -36,4 +36,13 @@ FoamFile
 #endif
 
 
+// Silly example, but can also force cast from scalar to bool
+
+#if #eval "bool(2 * pi())"
+    condition true;
+#else
+    condition false;
+#endif
+
+
 // ************************************************************************* //