Fixed a few bugs.

[rafaelbicudo]

1. The silhouette_score flag was missing flag in the io.py file.
2. There was an extra "t_opt".
3. Added a verification of the clust_opt.optimal_cut variable type, to avoid error when plotting the vertical line. Also changed to plot a single line instead of all lines in the multiple optimal threshold case.

[codecov-commenter]

Welcome to Codecov 🎉

Once merged to your default branch, Codecov will compare your coverage reports and display the results in this comment.

Thanks for integrating Codecov - We've got you covered ☂️