[Cython] Cython code producing different, incorrect results under Python 2.7 (not 3.x) under Cython 0.19

[Josh Warner]

Hi Stefan,

Every quantitative test is failing for _mcp.pyx (in
skimage.graph.tests.test_mcp.py), suggesting the problem is fairly global
to the MCP class. The three errors in skimage.graph.tests.test_spath.py are
due to incorrect MCP results which have no valid traceback path. No need to
investigate _spath.pyx.

The pure Python utility functions in _mcp.pyx appear to return correct
output, and I do not believe MCP.traceback is the culprit, rather, the
actual generated graph is incorrect. These errors appear in the MCP class,
so MCP_Geometric (subclass of MCP) is not the source.

Thus, I strongly suspect the problem lies within the MCP class, most likely
in MCP.find_costs(). Every failing test directly or indirectly uses this
method, and several of them directly test (and fail) on the returned cost
array.

Hopefully that helps,
Josh
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