Some ergonomic improvements for `Options`

[ZedThree]

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?

[ZedThree]

Also something like .update(const Options&)