Ability to Created Nested Groups

[rmabraham]

The current implementation allows an actor to have nested groups. Since a Group inherits from an Agent you can have a group containing nested groups.

This is against the spec:

• 4.1.2.2 An Anonymous Group MUST NOT contain Group Objects in the 'member' property.
• 4.1.2.2 An Identified Group MUST NOT contain Group Objects in the 'member' property.

[brianjmiller]

Noted, thanks for the issue. I don't know when we'll get to this, a PR following the current code style with unit tests always speeds up the process if you are looking to make sure this gets fixed in a timely manner :-).