Add hierarchical clustering section

[cimentadaj]

You didn't explain hierarchical clustering because there isn't enough time to teach it at the course. We still need a good section on the topic with a real world example.

[cimentadaj]

Students asked for this. And I found that the idea of subjectively choosing the k parameter made the method unattractive from a substantive point of view. I was thinking that aside from adding a section on hierarchichal clustering, it would be very handy to workout some strategies combining both things. That is, taking out a real world example, use HC for identifying clusters, cross-check this with k-means clustering and also with k-nearest neighbor, always using theoretical descriptions of the problem to assess whether the clusters make sense.