Add commutator functions

[Joseph-Edwards]

This PR adds bindings and tests for the functions commutator and add_commutator_rule.

There was a slight hiccup here, because Pybind11 binds C++ chars as Python strings (with a few extra steps). This was causing some issues with overload resolution for the functions with the optional id parameter. Specifically, does add_commutator_rule(p, x, y, z) mean "add the commutator rule $[x, y] = \epsilon$ with inverses given by z" or does it mean "add the commutator rule $[x, y] = z$ with inverses inferred from the presentation"? It is reasonable to want to be able to do either of these things, but difficult to express your intent to Python, because id has the same type as the other parameters.

To get around this, I've made the parameter id a keyword-only argument (see PEP 3102 for more info). Therefore, if we want to specify a non-standard identity, we need to explicitly provide it as a keyword argument (e.g. add_commutator_rule(p, x, y, id=z)). I've tried to make this clear in the doc, but if I'm happy to add more detail if it's needed.

I don't think we've added any keyword-only arguments before, so I needed to make a few tweaks to some of our scripts and helpers to accommodate this.

[james-d-mitchell]

Thanks @Joseph-Edwards, I'm happy with the keyword argument stuff you've proposed (in fact, in light of our recent conversation about this, I think maybe alphabet and inverses should be keyword args too. Obviously the CI failing needs to be fixed, but in principle I think this is good.