Added Spearman Correlation for similarities module.

[gautamramk]

Hi,
I have added a function to implement spearman correlation in the similarities module. I am very new to Open source contributions, please do bear with any errors.
I have followed the steps as in the Contribution Guidelines. If I am missing anything, do let me know. Also I am unaware on how to test my code. Would require some guidance.

[NicolasHug]

Thanks for the PR!

It looks pretty good even though I haven't checked it in details yet.

For the tests, take a look at how to use pytest and take inspiration from the test_similarities module.

It would also be helpful to have some simple benchmark comparing Spearman with other metrics in terms of RMSE and computation time

[gautamramk]

I'll run the tests, thanks a lot.

[NicolasHug]

This looks good and I'd like to merge it but could you please write a few tests in the test_similarities module?

You'll also need to update the AlgoBase.compute_similarities method.

[gautamramk]

I am a bit short of time due to my exams. Shall I write the tests after my exams? I shall modify the AlgoBase.compute_similarities now itself though.

[NicolasHug]

No worries there's absolutely no rush!
I just wanted to make sure you knew I need some tests before I can merge this ;)