presentation: attempt to resolve #391

[james-d-mitchell]

This resolves #391 by making std::vector<word_type> and std::vector<std::string> opaque types within present.cpp at least. The idea is that instead of such vectors being auto converted to lists, they are their own types.

PROS: p.rules return a reference, (at least on my machine) the tests pass, and it's possible to do p.rules[0] = something and this actually works as expected.

CONS: p.rules appears to be a list, but isn't, and so some things you might want to do with a list don't work (like index for example). This is a fairly large change.

I think the PROS out weigh the CONS, but happy to hear your opinion too @Joseph-Edwards

[james-d-mitchell]

I can't see why the MacOS CI jobs are failing, the logs don't provide enough info. It seems that libsemigroups_pybind11 builds okay, but then isn't loadable in python. I'm not going to investigate this further at this point.