Complexation of metal salts with macrocyclic polyethers in aprotic organic solvents. Crystal structure of ionic complexes [Co·15-crown-5·2CH3CN][CoCl3·CH3CN]2·CH3CN, [Co·15-crown-5·2CH3CN][CoCl4]·CH3CN and [Co·15-crown-5·2CH3CN][Co2Cl6]