Part 1: Move introduction of factors

[pssachdeva]

Factors are mentioned near the end of the lesson, after the introduction of data frames. This is likely to explain the stringsAsFactors parameter for data frames.

This is likely not necessary. First, stringsAsFactors can simply be omitted here: the default argument is FALSE, so it doesn't need to be discussed at all. It can be brought up in Part 2 if necessary.

The introduction of factors should occur with the other data types. It's a little odd that it's mentioned in the beginning of that section, but not referenced till much later in the lesson.