Thin Extrude: walls of a set thickness along sketch curves (#721) (#991)

Thin Extrude builds walls of a set thickness along sketch curves, open chains and closed loops, with the wall centred, inside, outside or on both sides at two thicknesses. Saved under format 20 (thin on an Extrude); an extrude without thin saves as before. Closes #721.

tools/oracle-analytic/cases_basic.py

@@ -515,6 +515,16 @@ def c29_two_sided_partial_revolve():
                   1, mp.v, A, mp.com(), [-20, -25, 0], [20, 10, 20], 12)
 
 
+def c82_thin_extrude_frame():
+    a, t_in, t_out, h = 40.0, 3.0, 1.0, 10.0
+    outer, inner = a + 2 * t_out, a - 2 * t_in
+    ring = outer * outer - inner * inner
+    return result("square frame: the lines' 40 mm square walled 3 mm inside and 1 mm outside (mitred corners) is "
+                  "42 x 42 less 34 x 34, 10 mm tall: V = (42^2 - 34^2) * 10, A = 2 (42^2 - 34^2) + 4 * 42 * 10 + 4 * 34 * 10",
+                  1, ring * h, 2 * ring + 4 * outer * h + 4 * inner * h, [0, 0, h / 2], [-outer / 2, -outer / 2, 0],
+                  [outer / 2, outer / 2, h], 10)
+
+
 CASES = {
     "01-box": c01_box,
     "02-cylinder": c02_cylinder,
@@ -542,6 +552,7 @@ CASES = {
     "26-sketch-belt": c26_sketch_belt,
     "27-sketch-trapezoid": c27_sketch_trapezoid,
     "29-two-sided-and-partial-revolve": c29_two_sided_partial_revolve,
+    "82-thin-extrude-frame": c82_thin_extrude_frame,
 }