Add more tips for the exercise

[GreatEmerald]

A lot of students got confused and spent a lot of time on data.frame subsetting; an example about how to get columns by name range (January:December) would be useful, and also about how to merge data.frames together (rbind/cbind and not merge()).

Many people created loops and lost time there, so it might be good to tell that no loops are necessary because extract() is already vectorised.

[GreatEmerald]

Done, perhaps good to add to the tutorial as per #10