Add more explanation to Transform(ation)s

[bencomp]

While working on changes for #157 I noticed that there is no context for transform(ation)s within the section "Transforming data" in episode 2 – it just starts doing it. I feel it would be helpful to explain or mention that:

• OpenRefine acts on all values in a column (or on those in selected rows)
• such actions are called transforms or transformations
• various common actions are available as menu items for direct access
• you can create more advanced transformations using GREL code (or another expression language)

This connects very much to #102, because the order in which we mention these points within the lesson matters. As I suggested in #102 we could use "Trim leading and trailing whitespace" as a first demonstration of transformations, perhaps after making the dataset messier (discussed in #108).

[bencomp]

I would be glad to receive suggestions for some narrative text about transformations.