Adding threshold parameter to f1

[SSaishruthi]

This PR closes #490

Changes:

1. Threshold parameter has been added to both F1 and F-Beta score.
2. Using the threshold parameter output is changed to 0 and 1.

[PhilipMay]

@SSaishruthi I do not think that this implementation with threshold fixes the problem.
Please see my comment here: #490 (comment)

[SSaishruthi]

Left a comment in the issue.

[SSaishruthi]

@PhilipMay

I am taking binary accuracy as a reference here: https://github.com/tensorflow/tensorflow/blob/2ff39d00faf8f7e433ddcae0aa278f6e573b0c55/tensorflow/python/keras/metrics.py#L630

In order for this to work you need to provide the threshold value as 0.49 above. I can probably fix the threshold default to be 0.5. I think below 0.5, it may not be considered as a good prediction.

I have this tested with scikit learn as well.https://colab.research.google.com/drive/1qSq0SsYkPqjdKUgM1RM4kKM67X75ocFj

Goal here is to make it compatible with both multi-class and multi-label

[Squadrick]

I'm closing this issue, I've created a new PR #502 that has threshold as well as some refactoring to f_scores and f_tests.