FGM - Added code for approximating the clustering coefficient
Fixed init to include my new function.
Added some tests. Updated style to match PEP-8.
Improved code and added docstrings.
Added proper tests.
Added new function to the documentation.
Generate random node indices instead of calling random.choice.
Merge pull request #934 from jtorrents/est-avg-clust-coeff

Add approximation for average clustering coefficient