Remove mutable default arguments in LinkStep

[riley-harper]

There's a discussion of mutable default arguments here: https://stackoverflow.com/questions/1132941/least-astonishment-and-the-mutable-default-argument. I didn't know that before! There are some empty lists passed as default arguments in the constructor for LinkStep. This isn't causing any issues right now, but it could cause issues in the future, as it gives many different places in the code unwitting write access to the same lists.

Let's replace the lists with tuples or do something similar to prevent the issue.

[riley-harper]

There are a few other places where there are mutable default arguments as well.

• hlink.linking.link_run.run_register_python()'s args argument ([])
• hlink.linking.link_run.run_register_sql()'s t_ctx argument ({})
• hlink.linking.core.comparison_features.get_features()'s pregen_features argument ([])
• Others?