In its quest to keep its DS3 popular, Citroën is now releasing a lovely little convertible. Press a button and the fabric roof retracts halfway, the whole length of the car - or even folds behind the back seats. We tested the only diesel-engine version, the eHDi 90 with 90 horsepower or 68 kilowatts of power. Will the model revive Citroën's slumping sales figures?