Agent grouping

[davidje13]

Add support for rendering a labelled box around a group of agents:

begin A, B, C
begin group of A, B: Foobar as G
A -> B
end G

Group beginnings should be pulled up to encompass agent creation, and group end pushed down to encompass agent destruction. Thought needs to be given to how this will interact with unexpected ordering, and how to maintain vertical alignment with ungrouped agents. This will probably behave in a similar manner to references. It may be desirable to allow notes and communication arrows referencing the group.