Manpower, junior size. Saturday's a holiday for most of the nation's small-fries, but to these youngsters of Roanoke, Virginia, it's fat collection day. As part of their junior commando activities, these boys and girls collect all fats and greases from local housewives during the week, and turn it over to school authorities on Saturday. From here, the fats are sent to a rendering plant where precious glycerin will be derived from it