A Dihalide–Decahydrate Cluster of [X 2 (H 2 O) 10 ] 2– in a Supramolecular Architecture of {[Na 2 (H 2 O) 6 (H 2 O@TMEQ[6])]·2(C 6 H 5 NO 3 )}X 2 (H 2 O) 10 (TMEQ[6] = α,α′,δ,δ′-Tetramethylcu