Crystal structure of hexa(N-acyl-3-hydroxy-2-naphthalenecarbo-hydrazide)- hexa(N,N-dimethylformamide)hexamanganese(III) — N,N-dimethylformamide (1:6), [Mn6(C13H9N2O3)6(C3H7NO)6] · (C3H7NO)6