Beneficial effects of substituting trivalent ions in the B-site of La<sub>0.5</sub>Sr<sub>0.5</sub>Mn<sub>1−x</sub>A<sub>x</sub>O<sub>3</sub> (A = Al, Ga, Sc) on the thermochemical generation of CO and H<sub>2</sub> from CO<sub>2</sub> and H<sub>2</sub>O