Classification chapter: reference to iris dataset is new to the reader

The chapter on Classification starts with the sentence "We have seen the iris data before". However, the iris dataset in the first chapter of [older lecture notes](http://file.biolab.si/notes/20-ul-ef-data-science-marketing.pdf) has now been replaced by a gene expression dataset, so the reader may not have seen it before.
By the way, the gene expression dataset is rather hard to understand for non-biologists. I would prefer a return to the iris dataset in the first chapter.

Read the Classification chapter and the first chapter referenced in the older lecture notes PDF, focusing on the opening reference to the iris dataset and the current gene expression dataset. Done means the chapter no longer assumes readers have seen iris data, and the introductory dataset choice matches the intended audience.

Written by the indexing model from the issue text.