case 4 air quality processing

[stijnvanhoey]

Some improvements:

• switcht eh figure of stack -> melt and remove the solution with stack to keep smooth flow
• replace the column_names = ['date'] + [item for pair in zip(hours, ['flag' + str(i) for i in range(24)]) for item in pair] flow by a chain(*zip(...)) to make it a bit more readable and easier to explain