A 3-D Organically Templated Mixed Valence (Fe2+/Fe3+) Iron Phosphate with Oxide-Centered Fe4O(PO4)4 Cubes: Hydrothermal Synthesis, Crystal Structure, Magnetic Susceptibility, and Mössbauer Spectroscopy of [H3NCH2CH2NH3]2[Fe4O(PO4)4]·H2O