Gravar-mail: catena-Poly[[[(1,10-phenanthroline-κ(2) N,N′)praseodymium(III)]-di-μ-4-hydroxy­benzoato-κ(4) O (1):O (1′)-μ-nitrato-κ(3) O,O′:O] bis­(1,10-phenanthroline)]