add sphere to the demos

demos/sphere.geo

+lc = .2;
+Point(1) = {0.0,0.0,0.0,lc};
+Point(2) = {1,0.0,0.0,lc};
+Point(3) = {0,1,0.0,lc};
+Circle(1) = {2,1,3};
+Point(4) = {-1,0,0.0,lc};
+Point(5) = {0,-1,0.0,lc};
+Circle(2) = {3,1,4};
+Circle(3) = {4,1,5};
+Circle(4) = {5,1,2};
+Point(6) = {0,0,-1,lc};
+Point(7) = {0,0,1,lc};
+Circle(5) = {3,1,6};
+Circle(6) = {6,1,5};
+Circle(7) = {5,1,7};
+Circle(8) = {7,1,3};
+Circle(9) = {2,1,7};
+Circle(10) = {7,1,4};
+Circle(11) = {4,1,6};
+Circle(12) = {6,1,2};
+Line Loop(13) = {-2,-8,10};
+Ruled Surface(14) = {13};
+Line Loop(15) = {10,3,7};
+Ruled Surface(16) = {15};
+Line Loop(17) = {-8,-9,1};
+Ruled Surface(18) = {17};
+Line Loop(19) = {-11,-2,5};
+Ruled Surface(20) = {19};
+Line Loop(21) = {5,12,1};
+Ruled Surface(22) = {21};
+Line Loop(23) = {-3,11,6};
+Ruled Surface(24) = {23};
+Line Loop(25) = {-7,4,9};
+Ruled Surface(26) = {25};
+Line Loop(27) = {4,-12,6};
+Ruled Surface(28) = {27};
+Surface Loop(29) = {-28,26,16,-14,20,24,-22,18};
+Volume(30) = {29};
+
+// try also netgen:
+// Mesh.Algorithm3D = 4;