Rewrite of 4.1.2 Actor

Large rewrite this time, plus some small changes:

Line 506:
Renamed 'anonymous group' to 'nameless group' to avoid confusion when reading the requirement that an anonymous group must contain a member property listing constituent agents. The term 'nameless group' makes more sense.

Lines 548 - 552
Adjusted Identified Group property table. There were several references to Agents, where probably Groups should be used. 

Please check for completeness and correctness before merge.