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	set g1 [new EventGroup $ns]
	$g1 add  $link0 $link1
	$ns at 60.0 "$g1 down"

See the new advanced tutorial section on event groups for a better
example.

Changed tbreport to dump the event groups table when in summary mode.
At the same time, I changed tbreport to use the recently added
virt_lans:vnode and ip slots, decprecating virt_nodes:ips in one more
place. I also changed the web interface to always dump the event and
event group summaries.

The parser gets a new file (event.tcl), and the "at" method deals with
event group events by expanding them inline into individual events
sent to each member. For some agents, this is unavoidable; traffic
generators get the initial params in the event, so it is not possible
to send a single event to all members of the group. Same goes for
program objects, although program objects do default to the initial
command now, at least on new images.

Changed the event scheduler to load the event groups table. The
current operation is that the scheduler expands events sent to a
group, into a set of distinct events sent to each member of the
group. At some point we proably want to optimize this by telling the
agents (running on the nodes) what groups they are members of.

Other News: Added a "mustdelay" slot to the virt_lans table so the
parser can tell assign_wrapper that a link needs to be delayed, say if
there are events or if the link is red/gred. Previously,
assign_wrapper tried to figure this out by looking at the event list,
etc. I have removed that code; see database-migrate for instructions
on how to initialize this slot in existing experiments. assign_wrapper
is free to ignore or insert delays anyway, but having the parser do
this makes more sense.

I also made some "rename" changes to the parser wrt queues and lans
and links. Not really necessary, but I got sidetracked (for several
hours!) trying to understand that rename stuff a little better, and
now I do.

it handles nodes, links, events(link_up, link_down)

it will _not_ work for arbitrary ns input files. The problem is that
an input file might say

set something [$ns some args]
$something do things

but my code doesn't return anything exciting in the first line.
I return Node, Link, Event, but if they are expecting an
Agent/whatever/whatever back, they are out of luck.
...and so there isn't anything for the second line to make do things
...and it'll break.