Add method in KNN to obtain nearest neighbors.
Internally, the apply now calls the new method to obtain the nearest
neighbors first and then computes the labels.
libshogun simple kNN example with random data
Use nearest neighbors method in classify for multiple k.
Also, avoid code duplication by introducing choose for multiple k
method, used in classify for multiple k w/ and w/o cover tree.
Merge pull request #1225 from iglesias/feature/knn_public_neighbors

Feature/knn public neighbors