Effect on insulin release of compounds structurally related to the potassium‐channel opener 7‐chloro‐3‐isopropylamino‐4H‐1,2,4‐benzothiadiazine 1,1‐dioxide (BPDZ 73): introduction of heteroatoms on the 3‐alkylamino side chain of the benzothiadiazine 1,1‐dioxide ring