Christmas Eve among the Bristlecone Pines   You traverse the snow-swept switchbacks following the trail of your Norse grandfather, a back country skier fascinated by the bristlecones, the ancient pines that shelter bighorns from the winds.   Far below Denver’s lights stretch away to Colorado Springs, and on Christmas Eve the traffic flow westward on I 70 brings winter pilgrims from as far away as Chattanooga.   In times past the liturgical day started at sunset, with the faithful gathering at a high place to give thanks while the heavens swung around the North Star, that celestial body your grandfather…