Add link to graph-tool

Created by: count0

What is this Python project?

Graph-tool is an efficient Python module for manipulation and statistical analysis of graphs (a.k.a. networks).

What's the difference between this Python project and similar ones?

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 it a level of performance that is comparable (both in memory usage and computation time) to that of a pure C/C++ library.

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README.md

@@ -1029,6 +1029,7 @@ Inspired by [awesome-php](https://github.com/ziadoz/awesome-php).
 * [Biopython](http://biopython.org/wiki/Main_Page) - Biopython is a set of freely available tools for biological computation.
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+* [graph-tool](https://graph-tool.skewed.de/) - Efficient manipulation, visualization and statistical analysis of graphs and networks.
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