Some ergonomic improvements for `Options`

A couple of things that would make `Options` even nicer to use:

- C++ iterator API: currently we have e.g. `getChildren()` which is `const`. Adding `const`/non-`const` `begin/end` would make the class more idiomatic
- Add `.units()`, `.longname()` etc for standard attributes
- Port Hermes-3 `getNonFinal` as e.g. `.getQuiet`?