fix bbox ref

Author: tomvanmele

src/compas/files/stl.py

@@ -357,7 +357,7 @@ def __init__(self, filepath, mesh, binary=False, solid_name=None, precision=None
     @property
     def _vertex_xyz(self):
         bbox = self.mesh.aabb()
-        xmin, ymin, zmin = bbox[0]
+        xmin, ymin, zmin = bbox.xmin, bbox.ymin, bbox.zmin
         if not self.binary and (xmin < 0 or ymin < 0 or zmin < 0):
             T = Translation.from_vector([-xmin, -ymin, -zmin])
             mesh = self.mesh.transformed(T)