Extend IPv4 forwarding example with p2p channel and csma channel

[Mingyumaz]

A solution to add p2p channel for p4-v1model-ipv4-forwarding example. This is an attempt to solve the #19

Here we are using a 100ms delay for a 1Gbps channel for test, with ./ns3 run p4-v1model-ipv4-forwarding -- --channelType=p2p --pktSize=1200 --appDataRate=1000Mbps --linkRate=1000Mbps --switchRate=120000 --linkDelay=100ms --flowDuration=0.1 --simDuration=10 --pcap=true
and get the result:

======================================
Final Simulation Results:
  ** [App Layer] (User configured DataRate)
  PDR: 100%
  TX: 12499200 bytes  (3.00001s -> 3.09999s,  elapsed=0.099984s)
  RX: 12499200 bytes  (3.20004s -> 3.30353s,  elapsed=0.103483s)
  TX Throughput: 1000.1 Mbps
  RX Throughput: 966.276 Mbps
  ** [MAC Layer]
  TX-host MacTx: 12936672 bytes  (3.00001s -> 3.1025s,  elapsed=0.10249s)
  RX-host MacRx: 12936672 bytes  (3.20004s -> 3.30353s,  elapsed=0.103483s)
  MAC TX Throughput: 1009.79 Mbps
  MAC RX Throughput: 1000.1 Mbps
======================================