add elliptic cone example

tutorials/ellipticcone.geo

@@ -0,0 +1,21 @@
+#
+## An elliptic cone
+#
+
+# An elliptic cone, given by the point c = (c x , c y , c z ) at the base of the cone along the main axis,
+# the vectors v and w of the long and short axis of the ellipse, respectively,
+# the height of the cone, h, and ratio of base long axis length to top long axis length, r:
+# ellipticcone (c x , c y , c z ; v x , v y , v z ; w x , w y , w z; h; r)
+
+# Note: The elliptic cone has to be truncated by planes similar to a cone or an elliptic cylinder.
+# When r =1, the truncated elliptic cone becomes an elliptic cylinder.
+# When r tends to zero, the truncated elliptic cone tends to a full elliptic cone.
+# However, when r = 0, the top part becomes a point(tip) and meshing fails!
+
+algebraic3d
+
+solid cutcone = ellipticcone ( 0, 0, 0; 5, 0, 0; 0, 2, 0; 5; 0.5)
+	and plane (0, 0, 0; 0, 0, -1)
+	and plane (0, 0, 5; 0, 0, 1);
+
+tlo cutcone;