wca-dimer example system

This pull request adds the wca-dimer system to reproduce the example found in the WESTPA manuscript. I've re-run this and cleaned up some stuff from the original run and everything looks good. I've also added a fairly extensive README file. 

Once this PR is in master, I'll add a link to view the ipynb through nbviewer.ipython.org so that there is a static rendering of the file. 

Feedback, especially on the README would be appreciated. 
Should this default to 0 for the more common case of a single GPU?