move for loop / list exercises from plotting episode 9 to for loops episode 12

[alee]

These will be removed after #302 is merged - see diff at

https://github.com/swcarpentry/python-novice-gapminder/pull/302/files

[davidrpugh]

@alee this issue appears to have been resolved. Can it be closed?

[alee]

I think we still need to add the accumulator exercise to Episode 12 actually (all the removed lines in ce7d704)

[davidrpugh]

I took a look at the accumulator pattern code that was deleted in Lesson 9. I don't think we should encourage the use of the accumulator pattern to solve the particular problem of getting a list of years. The Pandas idiomatic way to accomplish this is what we already demonstrate in Lesson 9.

# Instead of for loop we can use pandas dataframe functions to get the year.
years = data.columns.str.strip('gdpPercap_')

The only change I would advocate is a slight formatting change to emphasize the method chaining

years = (data.columns
             .str
             .strip('gdpPercap_'))