Tucson is on its way to becoming the world's center for straw-bale construction. By Kay Sather JUDY KNOX AND her husband Matts Myhrman say Tucson is on its way to becoming known world-wide as the center--if not quite the birthplace--of straw bale construction. This weekend Out on Bale, the straw bale education organization they founded here in 1991, will host a home tour and gathering of the movement's practitioners from all over the world. The first straw bale structures were built in Nebraska in the late 1800s by pioneers who considered them a temporary answer to their need for shelter in areas where wood, and sometimes even