Toward Ligand-Driven Light-Induced Spin Changing. Influence of the Configuration of 4 Styrylpyridine (stpy) on the Magnetic Properties of FeII(stpy)4(NCS)2 Complexes. Crystal Structures of the Spin-Crossover Species Fe(trans-stpy)4(NCS)2 and of the High-Spin Species Fe(cis-stpy)4(NCS)2