[CIG-SHORT] Strike-slip benchmark description updated

[Brad Aagaard]

Hi all:

I updated the description of the strike-slip benchmark with clearer text for 
the description of the boundary conditions. Charles Williams and I have 
worked out all of the major kinks and the results from PyLith and GeoFEST now 
agree very well with the analytic elastic solution and with each other for 
the viscoelastic solution. I updated the plots for the 500m resolution case, 
and will update the others when they become available. Note: The new plots 
use a different color scale and isosurface value because the discrepancies 
were so much smaller.

Brad