Daniel Rossen knew there was a lot to learn if he was finally going to finish his debut solo album. For nearly two decades, Rossen had been a crucial component of Grizzly Bear, the era-epitomizing act whose shared harmonies and interlaced textures meant he was responsible for only part of a whole.   But Rossen left the close-knit nest of Brooklyn many years ago, first for an isolated patch of land in upstate New York and then for the high desert climes of Santa Fe. The whole, as it were, was now his. So Rossen bought an upright bass (one of his instruments as a kid) and played all the parts himself, along with the cello. Best known as a guitarist, he took up woodwinds, too, buying several cheap student models and learning just enough to understand the rudiments. And then, largely at home in Santa Fe, he slowly built the world that is You Belong There, a riveting 10-song reintroduction to a voice that sounds both entirely familiar and fully reenergized by the act of unfettered expression.