Ch7 make the list of filenames separate lines #192

How could the content be improved?

In various parts in Ch7 we use for loops like this
for filename in ['data/2011_circ.csv', 'data/2012_circ.csv']:

It would be better to explicitly get a list of filenames like this
filenames = ['data/2011_circ.csv', 'data/2012_circ.csv']

My reasoning is that we've only just introduced for loops so it just makes it a bit easier to understand that we are looping through a list of filenames, and helping the learners learn exactly what the glob parts do