Refactoring of Os scope, next step

This series of commits refactors the handling of the Os objects further and merges the Os and OsScope classes.

There is still something left to do. The two main things are replacing SystemDescription.os_object by SystemDescription.os and the other one is a generic creation of scope objects when reading from disk. At the moment the Os scope is special-cased there.

But this should be a clean intermediate state. So I'm putting it up for review, so we can merge it to master and the refactoring branch doesn't diverge more.