[MRG] Add memory efficient implementation of NCA#99

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bellet merged 8 commits intoscikit-learn-contrib:masterfrom

wdevazelhes:fix/nca_perf

Jul 9, 2018

Commits on Jun 29, 2018

FIX Fixes #45 Add memory efficient implementation of NCA
William de Vazelhes
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ENH Add scipy optimizer lbfgs-b
William de Vazelhes
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Commits on Jul 2, 2018

FIX fix tests:
William de Vazelhes
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Commits on Jul 3, 2018

FIX: remove init parameter in NCA
William de Vazelhes
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FIX: remove random_state as well as unused imports
William de Vazelhes
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DOC: add docstring for NCA
William de Vazelhes
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Commits on Jul 4, 2018

TST: add more tests for edge cases and toy examples
William de Vazelhes
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STY: replace np.sum(array) by array.sum()
William de Vazelhes
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