Listing operators used in model

[mttjcksn]

Hey, thanks for Netron, it's a very convenient tool.

I'm testing some large models on various inferencing platforms and finding problems with not all model operators being supported - for example 'Erf' operator on ONNX.js.

It would be useful to be able to load a model into Netron and get a list of all operators it uses so I can cross-reference with the supported operators of the inferencing platform.

Thanks :)

[lutzroeder]

Duplicate of #204