move all models into a subdirectory

models/layouts/3x5box_layout.obj

models/layouts/box_animal_fine_layout.obj

models/layouts/box_animal_layout.obj

models/layouts/cube_layout.obj

models/layouts/cube_twisted_inset.obj

models/layouts/cuboid_twisted_inset.obj

models/layouts/cylinder_layout_prism.obj

models/layouts/hand_plain.obj

models/layouts/hand_plain_crooked_finger.obj

models/layouts/horse_layout.obj

models/layouts/horse_layout_praun_challenge.obj

models/layouts/spot_layout.obj

models/target-meshes/3x5box.obj

models/target-meshes/3x5box_6416.obj

models/target-meshes/cow_6938.obj

models/target-meshes/cube_933.obj

models/target-meshes/cuboid_16998.obj

models/target-meshes/cylinder.obj

models/target-meshes/cylinder_1966.obj

models/target-meshes/elephant_success.obj