Polymorphism in (n-C<sub>7</sub>H<sub>15</sub>NH<sub>3</sub>)<sub>2</sub>MnCl<sub>4</sub> and (n-C<sub>7</sub>H<sub>15</sub>NH<sub>3</sub>)<sub>2</sub>CdCl<sub>4</sub> as determined by heat-capacity measurements from 10 to 325 K