1,3-parallel O//O interactions in acyclic carbohydrates : the crystal and molecular structures of meso-D-glycero-L-altro-heptitol and the monohydrate, meso-D-glycero-L-ido-heptitol, and meso-D-glycero-L-altro-heptitol and D-glycero-L-galacto-heptitol heptaacetates