show how to connect volumes to boundary layer surfaces

benchmarks/extrude/sphere_boundary_layer.geo

@@ -41,4 +41,43 @@ Extrude {
   Surface{14:28:2};  Layers{10, 0.2}; // Recombine; 
 }
 
+Point(100) = {-1.5,-1.5,-1.5, lc};
+Point(101) = {-1.5,1.5,-1.5, lc};
+Point(102) = {1.5,-1.5,-1.5, lc};
+Point(103) = {1.5,1.5,-1.5, lc};
+Point(104) = {-1.5,-1.5,1.5, lc};
+Point(105) = {-1.5,1.5,1.5, lc};
+Point(106) = {1.5,-1.5,1.5, lc};
+Point(107) = {1.5,1.5,1.5, lc};
+Line(165) = {100, 102};
+Line(166) = {102, 103};
+Line(167) = {103, 101};
+Line(168) = {101, 100};
+Line(169) = {100, 104};
+Line(170) = {104, 106};
+Line(171) = {106, 102};
+Line(172) = {101, 105};
+Line(173) = {105, 107};
+Line(174) = {107, 103};
+Line(175) = {105, 104};
+Line(176) = {106, 107};
+Line Loop(177) = {167, 168, 165, 166};
+Plane Surface(178) = {177};
+Line Loop(179) = {166, -174, -176, 171};
+Plane Surface(180) = {179};
+Line Loop(181) = {171, -165, 169, 170};
+Plane Surface(182) = {181};
+Line Loop(183) = {176, -173, 175, 170};
+Plane Surface(184) = {183};
+Line Loop(185) = {172, 175, -169, -168};
+Plane Surface(186) = {185};
+Line Loop(187) = {172, 173, 174, 167};
+Plane Surface(188) = {187};
 
+Surface Loop(1000) = {20, 22, 18, 14, 26, 16, 24, 28};
+Surface Loop(1001) = {45, 96, 113, 79, 62, 130, 147, 164};
+Surface Loop(1002) = {188, 186, 184, 180, 178, 182};
+Volume(1000) = {1000}; // inside
+Volume(1001) = {1002, 1001}; // outside
+
+Mesh.Algorithm3D = 4;