* Turns out Darknet's bounding box co-ordinates refer to the center of
the bounding box, and that width and height refers to the bounding
box's dimensions.
* Use `C.CString()` instead of `C._GoStringPtr()`.
- Seems like sometimes the pointer returned by `C._GoStringPtr()`
could already be freed, while work is being done in C code that uses
the pointer. Using `C.CString()` ensures the C string's lifetime.
* Remove own implementation of `free_network()`; Darknet actually does
have its own!
* Changed some field names of `YOLONetwork` to make them look more
consistent.
