Chefs are reinventing the taco,seeking a truer Mexican cuisine than that offered by stodgy beans and melted yellow cheese. One uniquely Mexican ingredient helping their quest is chipotle in adobo,a rich,dark,smoky,spicy sauce of smoke-dried,ripe jalapeno chillies. With it,we can add smoke and fire to the simplest of dishes. Like this fabulous chicken tinga of shredded chicken and onions simmered in chipotle and tomato sauce,ready to serve with tortillas,tacos,tosta