Improved error handling in many places to detect missing files or NULL file handles.
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close() file before dying.
reduced the execution time of the autoencoders notebook
add the notebook
removed eigen3/viennacl includes from some class headers
Reduced accuracy of CustomKernel.sum_block unit-test

To prevent failure on Travis buildbot OSX caught in job
https://travis-ci.org/shogun-toolbox/shogun/jobs/32569634
Merge pull request #2494 from lambday/develop

Reduced accuracy of CustomKernel.sum_block unit-test
Fix failure of KNN with cross-validation and memory leak on Features copy constructor.
added cpp examples for neuralnets
fixed defects in graph cuts
changes for peer review
remove the eigen3 header files
Merge pull request #2453 from Jiaolong/fix_defects

fixed defects in graph cuts
Scene classification notebook added
notebook polished
added check for opencv version