Some Python tests fail for higher number of threads

[fabratu]

When executed with 48 threads, several unittests for the Python interface fail:

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FAIL: testApproxElectricalCloseness (test_centrality.TestCentrality.testApproxElectricalCloseness)
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Traceback (most recent call last):
  File "/home/brandtfa/scripts/test/test_centrality.py", line 55, in testApproxElectricalCloseness
    self.assertLessEqual(abs(apx[u] - pinv[u]), eps)
AssertionError: 0.11044452053538456 not less than or equal to 0.1

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FAIL: testApproxSpanningEdge (test_centrality.TestCentrality.testApproxSpanningEdge)
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Traceback (most recent call last):
  File "/home/brandtfa/scripts/test/test_centrality.py", line 68, in testApproxSpanningEdge
    self.assertLessEqual(abs(apxScore - exactScore), 2*eps)
AssertionError: 0.24007940334904146 not less than or equal to 0.2

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FAIL: testForest (test_centrality.TestCentrality.testForest)
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Traceback (most recent call last):
  File "/home/brandtfa/scripts/test/test_centrality.py", line 292, in testForest
    self.assertLessEqual(abs(apx - exact), eps)
AssertionError: 0.06750794287494666 not less than or equal to 0.05

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Reducing the number of threads mitigates this problem.