By Dominique Vidalon PARIS (Reuters) -Shares in supermarket group Casino slumped on Monday after the company cut the 2021 profit outlook for its French retail operations, igniting investor worries over its ability to boost cashflow in a tough domestic market. Casino blamed the downgrade on a steeper-than-expected decline in France’s food market in the fourth quarter, notably in the Paris region, where it has a large presence through its high-end Monoprix stores, a key profit contributor. By midday Casino’s shares were down 13%. Its woes weighed on its larger domestic rival Carrefour, whose sha…