Made .size of hough_transform a property rather than a function.

tools/python/src/image2.cpp

@@ -189,7 +189,7 @@ ensures \n\
 
     py::class_<hough_transform>(m, "hough_transform", class_docs)
         .def(py::init<unsigned long>(), doc_constr, py::arg("size_"))
-        .def("size", &hough_transform::size,
+        .def_property_readonly("size", &hough_transform::size,
             "returns the size of the Hough transforms generated by this object.  In particular, this object creates Hough transform images that are size() by size() pixels in size.")
         .def("get_line", &ht_get_line, py::arg("p"),
 "requires \n\