Synthesis of C- and N-Functionalised Derivatives of 1,4,7-Triazacyclononane-1,4,7-triyltriacetic Acid (NOTA), 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetrayltetraacetic Acid (DOTA), and Diethylenenetriaminepentaacetic Acid (DTPA): Bif