Chelation of CeIII and PrIII by N,N′-Bis(pyridoxylideneiminato)ethylene, a Schiff Base Ligand Derived from Vitamin B6: Synthesis, Structural Features and TGA Evaluations of [M(Hpyr2en)(pyr2en)]·nH2O (M = CeIII, n = 2; PrIII, n = 8)†