Add 68-Bus System

[ccoffrin]

Explore the possibility of including a network derived from the 68-Bus, 16-Machine, 5-Area Dynamic Test System.

Related Links:

• http://sites.ieee.org/pes-psdp/benchmark-systems-2/
• https://electricgrids.engr.tamu.edu/electric-grid-test-cases/
• http://icseg.iti.illinois.edu/new-england-68-bus-test-system/
• http://sites.ieee.org/pes-resource-center/files/2015/08/PES_TR18_Benchmark-Systems-for-Small-Signal-Stability-Analysis-and-Control.pdf
• http://www.sel.eesc.usp.br/ieee/NETS68/New_England_New_York_68_Bus_System_study_report.pdf