Preprocessing Notebook #1 Time issue for Challenge 2

[joseaveldanes]

We should have more time to address the functions that happen in challenge 2. The participants need a bit more time to understand what each function does so they can piece together a solution. For example, it is not clear what .extend() and os.path.join() functions right off the bat. Relatedly--the addition of the for-loop I think was challenging for some of the students. I think generally people just need more time, they can get there but we need ot give it more time. @rbarreto