Enthalpy of formation for indazoles (indazole, 1H‐indazole‐3‐carboxylic acid, 1H‐indazole‐5‐carboxylic acid, 1H‐indazole‐6‐carboxylic acid and 1‐methyl‐1H‐indazole‐6‐carboxylic methyl ester): experimental and theoretical studies