Create class hierarchy for AnnotationSet

[johann-petrak]

Consider:

• AnnotationSet is an abstract base class
• DetachedAnnotationSet is the class used for detached sets
• AttachedAnnotationSet is the class used for in-document sets

We could just use the API we have now for AnnotationSet for both, but ideally refactor:

• anything related to logging is only in AttachedSet
• constructor only possible for DetachedSet

Advantages:

• easier to see with print etc. what set we are dealing with
• clearer what each type of set can do
• can be used with type hinting to document which methods return which sets