Get rid of base_object::get_values

[joesiltberg]

It seems like a risky construct since it returns a reference into the map in the object, what happens if we modify the base_object while we hold on to that reference?

If the current behaviour is needed somewhere because of performance perhaps we should have a safe and an unsafe version, where the unsafe version has a name that makes it clear that what we get back is a reference that might not be valid under all circumstances...