add elasticity reaction force computing example

examples/reaction-force-computing/disk.geo

@@ -0,0 +1,27 @@
+// Choose the mesh size:
+c = 0.1;
+
+// Number of extrude layers:
+n = 3;
+
+Point(1) = {0, 0, 0, c};
+Point(2) = {1, 0, 0, c};
+Point(3) = {0, -1, 0, c};
+Point(4) = {0, 1, 0, c};
+Point(5) = {-1, 0, 0, c};
+
+Circle(1) = {5, 1, 4};
+Circle(2) = {4, 1, 2};
+Circle(3) = {2, 1, 3};
+Circle(4) = {3, 1, 5};
+
+Line Loop(5) = {1, 2, 3, 4};
+Plane Surface(6) = {5};
+Recombine Surface{6};
+
+Extrude{0,0,0.1}{Surface{6}; Layers{n}; Recombine;}
+
+Physical Volume(1) = {1};
+Physical Surface(2) = {15,19,23,27};
+Physical Surface(3) = {28};
+Physical Line(4) = {8,9,10,11};