I was told to come alone. I was not to carry any identification, and would have to leave my cell phone, audio recorder, watch, and purse at my hotel . . .      For her whole life, Souad Mekhennet, a reporter for the Washington Post who was born and educated in Germany, has had to balance the two sides of her upbringing - Muslim and Western. She has also sought to provide a mediating voice between these cultures, which too often misunderstand each other.