Example of CustomKernel with IndexFeatures in libshogun.
fix function name.
Add IndexFeature to Python interface.
Remove the print function.
Add new example for index feature in custom kernel.
Modify example output, and output submatrix in python example.
Modify the shogun-data.
Merge pull request #2167 from pl8787/indexfeatures

Example of CustomKernel with IndexFeatures in libshogun.