Create bw_monitor_13.py

To run:

sudo mn --topo single,3 --mac --switch ovsk --controller remote -x

in s1 xterm, do:
ovs-vsctl set bridge s1 protocols=OpenFlow13

in c0 xterm, do:
go to the folder that bw_monitor_13.py is located in
ryu-manager --verbose ./bw_monitor_13.py