Python nethandles

[xchang1]

This updates libhandlegraph to make equality of net_handle_t's show up in the python bindings. Update of #190

[xchang1]

This also adds stuff from #223

[adamnovak]

It looks like the way #223 is trying to generate bindings for std::unordered_set can't work, because those bindings end up being specific to the STL implementation thy were generated on (mentioning types like std::__detail::_Node_iterator<std::string, true, true>) and can't be committed to the repo and expected to build on other platforms.

If we can't convince Binder to generate bindings that work for everyone's std::unordered_set, we might need to try Binder's --include-pybind11-stl option from https://cppbinder.readthedocs.io/en/latest/config.html to get it to do some magic for converting these containers to/from Python containers, instead of binding them normally.