Hydrothermal Syntheses and Characterization of Novel 3D Open-Framework and 2D Grid Lanthanide Fumarates: Ln2(fum)3(H2fum)(H2O)2 (Ln = Ce or Nd), [Sm2(fum)3(H2O)4](H2O)3, and [Yb2(fum)3(H2O)4](H2O)2