Structure–Activity Relationships of New 1‐substitutedmethyl‐4‐[5‐(N‐methyl‐N‐propylamino)pentyloxy]piperidines and Selected 1‐[(N‐substituted‐N‐methyl)‐3‐propyloxy]‐5‐(N‐methy‐l‐N‐propyl)‐pentanediamines as H3‐Antagonists