Improve tests failing due to round off error/finite precision

[jameskermode]

Thanks to Pétur Bryde for reporting

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FAIL: test_to_atoms_frac_pos (test_castep.TestCastepCell)
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Traceback (most recent call last):
  File "/home/petur/prog/QUIP/tests/test_castep.py", line 198, in test_to_atoms_frac_pos
    self.assertEqual([x.split() for x in c['POSITIONS_FRAC']], [x.split() for x in c2['POSITIONS_FRAC']])
  File "/home/petur/prog/QUIP/tests/quippytest.py", line 62, in assertEqual
    self.fail('%s != %s' % (a,b))
AssertionError: [['O', '0.414382', '0.259186', '0.792179'], ['O', '-0.259186', '0.155196', '1.458845'], ['O', '-0.155196', '-0.414382', '1.125512'], ['O', '0.259186', '0.414382', '-0.792179'], ['O', '-0.414382', '-0.155196', '-0.125512'], ['O', '0.155196', '-0.259186', '-0.458845'], ['Si', '0.474452', '0.000000', '0.666667'], ['Si', '0.000000', '0.474452', '0.333333'], ['Si', '-0.474452', '-0.474452', '0.000000']] != [['O', '0.414382', '0.259186', '0.792179'], ['O', '-0.259186', '0.155196', '1.458845'], ['O', '-0.155196', '-0.414382', '1.125512'], ['O', '0.259186', '0.414382', '-0.792179'], ['O', '-0.414382', '-0.155196', '-0.125512'], ['O', '0.155196', '-0.259186', '-0.458845'], ['Si', '0.474452', '-0.000000', '0.666667'], ['Si', '-0.000000', '0.474452', '0.333333'], ['Si', '-0.474452', '-0.474452', '0.000000']]

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FAIL: testminim (test_potential.TestPotential_ElasticMinim)
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Traceback (most recent call last):
  File "/home/petur/prog/QUIP/tests/test_potential.py", line 5064, in testminim
    self.assertArrayAlmostEqual(self.at.lattice, self.lat_ref)
  File "/home/petur/prog/QUIP/tests/quippytest.py", line 91, in assertArrayAlmostEqual
    self.fail('Maximum abs difference between array elements is %e at location %r' % (absdiff.max(), loc))
AssertionError: Maximum abs difference between array elements is 4.937909e-07 at location [3, 3]

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FAIL: test_md_velo.sh (test_shell_scripts.Test_TopLevel)
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Traceback (most recent call last):
  File "/home/petur/prog/QUIP/tests/test_shell_scripts.py", line 47, in run_script
    self.fail()
AssertionError: None