Fix failing unit test in Python package

[kordejong]

python/algorithm/space/focal/test/python_test.cc

======================================================================
FAIL: test_pcraster_example (slope_test.SlopeTest)
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Traceback (most recent call last):
  File "slope_test.py", line 46, in test_pcraster_example
    self.assertMaskedRasterEqual(raster_we_got, raster_we_want)
  File "/home/kor/development/objects/Debug/fern/bin/fern/test/test_case.py", line 70, in assertMaskedRasterEqual
    values_we_want[row][col], row, col))
AssertionError: 2.6584508419 != 2.65845108032 for cell 4, 3

----------------------------------------------------------------------
Ran 1 test in 0.023s

FAILED (failures=1)
Running 1 test case...
/home/kor/development/projects/fern/source/fern/python/algorithm/space/focal/test/python_test.cc(18): error in "unit_test": check result == 0 failed [256 != 0]

*** 1 failure detected in test suite "fern fern_python_extension_algorithm_space_focal"