PI queue disc implementation for ns-3

PI controller [1] is a queuing discipline, proposed as an alternative to RED.

This patch provides an implementation of PI controller in ns-3 along with documentation, test-suite and example program.

Your reviews are much required.

Thanks,
Viyom Mittal

[1] C. V. Hollot, Vishal Misra, Don Towsley and Wei-BoGong, "On Designing Improved Controllers for AQM Routers Supporting TCP Flows", IEEE/INFOCOM, 2001.

[Viyom Mittal, 2017-03-16 06:29:44]

I have modified the PI code (Patch 2) to incorporate the recent modifications in
ns-3 to use queue as a template.

Thanks and Regards,
Viyom Mittal

[Viyom Mittal, 2017-11-28 15:01:36]

I have modified the PI code (Patch 4) to support ECN marking and use stats from
QueueDisc class.

Thanks and Regards,
Viyom Mittal