Crystal structure of bis(μ ₂-1,5-bis[(E)-1-(2-hydroxyphenyl)ethylidene] thiocarbonohydrazide)-bis(dimethylformamide)-dizinc(II) dimethylformamide solvate, C₄₀H₄₆N₁₀O₆S₂Zn₂⋅C₃H₇NO