silhouette_analysis plot improvements

[edublancas]

here are some things we can improve in the silhouette_analysis

• silhouette_analysis_from_results should not be part of the public API since it's method that it's used as an intermediate step, so we should rename it to something like _silhouette_analysis. I think this computes silhouettes for a single cluster so maybe _silhouette_analysis_one_cluster

• silhouette_analysis had a hardcoded figure size (which I already removed); however, to make the tests pass, I had to move this hardcoded figure size to the tests, we should remove this

[neelasha23]

silhouette_analysis_from_results was added to be consistent with elbow_curve_from_results (user wants to generate plots from custom model). Do we need to make elbow_curve_from_results internal too?