Fix overwriting of some FACS results

This pull request fixes a problem on performance.py that was overwriting some FACS results, concretely the results for tests ran with 1 and 8 threads.

Take a look at @brainstorm 's notebook executed with this changes :-)

http://nbviewer.ipython.org/urls/dl.dropboxusercontent.com/u/3046000/benchmarks_facs.ipynb?create=1