Documentation on CMFD Theory

This PR adds information about the theory of CMFD and how it is implemented into OpenMC. Note, I use the sphinxcontrib-tikz extension. In order to obtain checkout the source using:

git clone https://bitbucket.org/philexander/tikz.git

Then do ```sudo python setup.py install```. Note, if you download this extension via pip it will not have the directive ```:include:``` implemented. Hopefully, we can use this extension for documentation, it seems to work fairly well. 

This PR also addresses issue #222.
"XS" and "DC" never defined in flow chart