[exercise-grids] Add .geo script for the structured grid.

exercises/exercise-grids/grids/grid_structured.geo

+X = 60;
+Y = 40;
+Res = 1;
+
+Point(1) = {0,0,0,Res};
+Point(2) = {X,0,0,Res};
+Point(3) = {X,Y,0,Res};
+Point(4) = {0,Y,0,Res};
+
+Line(5) = {1,2};
+Line(6) = {2,3};
+Line(7) = {3,4};
+Line(8) = {4,1};
+
+Line Loop(9) = {5,6,7,8};
+
+Plane Surface(10) = 9;
+
+numCellsX = 24;
+numCellsY = 16;
+
+Transfinite Line{5} = numCellsX + 1;
+Transfinite Line{6} = numCellsY + 1;
+Transfinite Line{7} = numCellsX + 1;
+Transfinite Line{8} = numCellsY + 1;
+
+Transfinite Surface "*";
+Recombine Surface "*";
+Transfinite Volume "*";
+