Reworked the core examples to exclude contributed examples.

examples/Ex017_Shelling_to_Create_Thin_Features.py

@@ -0,0 +1,14 @@
+import cadquery as cq
+
+# Create a hollow box that's open on both ends with a thin wall.
+# 1.  Establishes a workplane that an object can be built on.
+# 1a. Uses the named plane orientation "front" to define the workplane, meaning
+#     that the positive Z direction is "up", and the negative Z direction
+#     is "down".
+# 2.  Creates a plain box to base future geometry on with the box() function.
+# 3.  Selects faces with normal in +z direction.
+# 4.  Create a shell by cutting out the top-most Z face.
+result = cq.Workplane("front").box(2, 2, 2).faces("+Z").shell(0.05)
+
+# Displays the result of this script
+show_object(result)