remove declaration of private properties in classes

[dave-doty]

When I wrote fields such as Domain._name, I didn't realize this was a way to handle running code on field access:

https://python-course.eu/oop/properties-vs-getters-and-setters.php

i.e., just have an undeclared private field, while declaring a public "field" that's really implemented as a property. This will clean up the API a bit.