Enthalpies of formation of piperazinium dinitrate, C<sub>4sub>H<sub>12sub>N<sub>2sub>(NO<sub>3sub>)<sub>2sub>, dichloride, C<sub>4sub>H<sub>12sub>N<sub>2sub>Cl<sub>2sub>, and dichloride monohydrate, C<sub>4sub>H<sub>12sub>N<sub>2sub>Cl<sub>2sub>·H<sub>2sub>O