point-to-point helper for parking lot topology

This patch provides a helper for simulating parking lot topology (or multiple bottleneck topology) using point-to-point links.

This type of topologies are used for evaluating the performance of TCP [1].

- point-to-point-parking-lot{.h, .cc} are added to src/point-to-point-layout module.

- An example program parking-lot-animation.cc has been added to src/netanim/examples 

[1] https://tools.ietf.org/html/draft-irtf-iccrg-tcpeval-01