Added subset stack in feature selection

Needed for keeping track of selected features which is required in a
classification scenario for unseen data.
added functions in SGNDArray
move doc/md into git@github.com:shogun-toolbox/shogun.wiki.git submodule
added binary denoising notebook
Merge pull request #2490 from Jiaolong/nd_array

added functions in SGNDArray
Merge pull request #2467 from Jiaolong/nb_binary_denosing

added binary denoising notebook
Fix submodule repository type of shogun.wiki
update data-checkout to 24b8341
Merge pull request #2532 from lambday/develop

Added subset stack in feature selection
Added dummy main in variational approximation example

Fixes compilation error (reproduced with macosx 10.9 and its default compiler)
fixed #2536