Statistics of 4H-1,2,4-Pyridothiadiazine 1,1-dioxides and 2,3-dihydro-4H-1,2,4-pyridothiadiazine 1,1-dioxides chemically related to diazoxide and cyclothiazide as powerful positive allosteric modulators of (R/S)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid receptors: design, synthesis, pharmacology and structure-activity relationships