imgTofloat32 + fill_image_f32 took more than double the time since tofloat created needless allocations and iterate over each pixel, and fill_image in c then iterate over each pixel*3 again just to move it into the c image.
* 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.