add tf_ring

src/experimental/tf_ring.scad

@@ -0,0 +1,10 @@
+use <rotate_p.scad>;
+use <experimental/tf_y_twist.scad>;
+
+function tf_ring(size, point, radius, angle, twist = 0) = 
+    let(
+        yleng = size[1],
+        a_step = angle / yleng,
+        twisted = tf_y_twist(size, point, twist)
+    )
+    rotate_p([radius + twisted[0], 0, twisted[2]], a_step * twisted[1]);