Studies to Investigate the in Vivo Therapeutic Window of the γ-Secretase Inhibitor N²-[(2S)-2-(3,5-Difluorophenyl)-2-hydroxyethanoyl]-N¹-[(7S)-5-methyl-6-oxo-6,7-dihydro-5H-dibenzo[b,d]azepin-7-yl]-l-alaninamide (LY411,575) in the CRND8 Mouse