- Fat Tree readme now includes documentation for the custom- "pruned"
fat tree layout which is used to create the Summit network.

Documentation in src/networks/model-net/doc/README.fattree.txt

This commit is joint work by Jens Domke and Nikhil Jain.

Jens did all the ground work for dumping the topology and reading the routing
tables. He also wrote the scripts that perform the intermediate steps that
generate the routing tables using the topology.

Nikhil integrated Jens's changes into CODES by breaking them into a 3 step
process as described in src/networks/model-net/doc/README.fattree.txt.

Change-Id: I1478fb2ab83b9ded0673d8f897e09a423868795a