maxpool node

[gillmac13]

@Rapternmn
I wonder how difficult it would be to integrate a maxpool node in the ONNX conversion. That would enable converting yolov3-spp and yolov3-tiny models which are also very popular.

[gillmac13]

@Rapternmn
OK, don't worry..
The original "yolov3_to_onnx.py" by Nvidia provides all the necessary nodes, including maxpool. I'll give it a try and make a yolov3-spp model in ONNX and RT formats.
Thanks for your work!