Add a new formula for graph-tool

graph-tool is an efficient python module for manipulation and statistical analysis of graphs (a.k.a. networks). Contrary to most other python modules with similar functionality, the core data structures and algorithms are implemented in C++, making extensive use of template metaprogramming, based heavily on the Boost Graph Library. This confers a level of performance which is comparable (both in memory usage and computation time) to that of a pure C++ library.