Gravar-mail: catena-Poly[[diaqua­[1-ethyl-6-fluoro-4-oxo-7-(piperazin-1-yl)-1,4-dihydro­quinoline-3-carboxyl­ato-κ(2) O,O′]nickel(II)]-μ-4,4′-oxydibenzoato-κ(2) O:O′]