ENH Add sample_indices for SMOTE class

[tianlinhe]

Reference issue #772

What does this implement/fix? Explain your changes.

To add the sample indices of the generating samples that do not belong to the original dataset.

• sample_indices() exposes the index of the original samples in a list
• arg get_which_neighbors=True reveals from which random neighbor the sample was generated. For original data it returns 0, which means itself

Any other comments?

-[x] Changes pass PEP8 (no issue on the changed lines)

It is only implemented in SMOTE. If needed I am happy to include it in ADASYN.

[pep8speaks]

Hello @tianlinhe! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file imblearn/over_sampling/_smote/base.py:

Line 303:35: E226 missing whitespace around arithmetic operator
Line 330:1: W293 blank line contains whitespace