1) IP address based routes can now be added
   - The IP address is set on a link object
   - An "$ns rlink" is used to instantiate links
     that get cut and cross physical partitions
   - Traffic agents that are across physical
     partitions (i.e. different instances of nse)
     are connected by a new "$ns ip-connect"
     mechanism
   - A new Node instproc "add-route-to-ip" adds
     IP address based routes.
   - Changed ns multicast addressing to use 3 bits
     instead of the default 1
   - Currently, the classifier does a lookup on a
     complete 32 bit IP and if a target to route to
     is not found, uses a 24 bit IP mask. It does not
     try to match IP prefixes of all lengths. I'll add
     that later if necessary
2) NS packets that cross partitions are encapsulated in
   IPPROTO_ENCAP IP packets.
3) RAW IP sockets used to inject packets into the network
   now use a rtabid paramater so that packets can be
   routed according to different routing tables

Tested with 2 test cases, one with UDP/CBR traffic
and another with default NS TCP/FTP traffic. Setup was done
manually. As I do testbed integration, there may be more changes.
Here's the test setup:

   2.2    2.3   1.2      1.3   3.2      3.3
n0 --------- n1 ----------- n2 ------------ n3

n0,n1 are on one physical node and n2,n3 are on another. The n1-n2 link
is cut.

A TCP example:

---------------------physnode0---------------------
set ns [new Simulator]
$ns use-scheduler RealTime

set n0 [$ns node]
set n1 [$ns node]

$ns duplex-link $n0 $n1 10Mb 5ms DropTail
[$ns link $n0 $n1] set-ip 10.1.2.2
[$ns link $n1 $n0] set-ip 10.1.2.3

set rl0 [$ns rlink $n1 10.1.1.3 2Mb 40ms DropTail]
$rl0 set-ip 10.1.1.2

set tcp0 [new Agent/TCP]
# The last parameter specifies the port
$ns attach-agent $n0 $tcp0 20
$ns ip-connect $tcp0 10.1.3.3 20
set ftp0 [new Application/FTP]
$ftp0 attach-agent $tcp0

$n0 add-route-to-ip 10.1.3.3 10.1.2.3
$n1 add-route-to-ip 10.1.3.3 10.1.1.3

$ns at 1.0 "$ftp0 start"
$ns at 10.0 "$ftp0 stop"
-----------------end physnode0---------------------

---------------------physnode1---------------------
set ns [new Simulator]
$ns use-scheduler RealTime

set n2 [$ns node]
set n3 [$ns node]

$ns duplex-link $n2 $n3 10Mb 5ms DropTail
[$ns link $n2 $n3] set-ip 10.1.3.2
[$ns link $n3 $n2] set-ip 10.1.3.3

set rl1 [$ns rlink $n2 10.1.1.2 2Mb 40ms DropTail]
$rl1 set-ip 10.1.1.3

set tcpsink0 [new Agent/TCPSink]
$ns attach-agent $n3 $tcpsink0 20
$ns ip-connect $tcpsink0 10.1.2.2 20

$n3 add-route-to-ip 10.1.2.2 10.1.3.2
$n2 add-route-to-ip 10.1.2.2 10.1.1.2
-----------------end physnode1---------------------