added the *_corner() routines to rectangle and drectangle and made these
objects constructable from instances of each other.

randomly_color_image(), jet(), skeleton(), find_line_endpoints(), get_rect(), shrink_rect(),
grow_rect(), and image_gradients.

- Moved all uses of PYBIND11_MAKE_OPAQUE to a common header to avoid possilbe ODR violations.
- Added python binding for make_bounding_box_regression_training_data().

* Replace boost::python with pybind11

* Replace add_python_module with pybind11_add_module

* Fix clang error on type-dependent expression

Easy change to allow comparisons on the Python side for
both rectangle and drectangle

The correlation tracker deals with the drectangle
(double rectangle) class which currently isn't wrapped. Therefore,
I add the drectangle class and refactor rectangles into their
own file. I also added a load of methods on rectangle that might
be useful such as intersection/contains/area etc.