Characterization of two geometrical isomers of (1,3-propanediamine-N,N'-diacetato-N,N'-di-3-propionato)nickelate(II). X-ray structure of the binuclear complex trans(O5)-[Ni2(1,3pddadp)(H2O)4].4H2O and octahedral distortion of edta-type chelates