Adding function to recombine mesh for generation of quad elements (#305)

* adding set_recombined_surfaces in geometry.py for generation of quad meshes and adding corresponding test in test_recombine.py. Related to issue #304

* reformatting test_recombine.py to pass ci/lint checks:

*  adding whitespace around arithmetic operator as indicated by ci/lint tests

* using black to reformat

* reformatting test_recombine.py

pygmsh/built_in/geometry.py

@@ -258,6 +258,17 @@ def set_transfinite_surface(self, surface, size=None, orientation=None):
         self._GMSH_CODE.append(code + ";")
         return
 
+    def set_recombined_surfaces(self, surfaces):
+        for i, surface in enumerate(surfaces):
+            assert isinstance(
+                surface, (PlaneSurface, Surface)
+            ), "item {} is not a surface".format(i)
+        code = "Recombine Surface {{{}}}".format(
+            ", ".join([surface.id for surface in surfaces])
+        )
+        self._GMSH_CODE.append(code + ";")
+        return
+
     def add_circle(
         self,
         x0,

test/test_recombine.py

@@ -0,0 +1,34 @@
+import pygmsh
+import numpy as np
+
+
+def test():
+    geom = pygmsh.built_in.Geometry()
+    p0 = geom.add_point((0.0, 0.0, 0.0), lcar=1.0)
+    p1 = geom.add_point((2.0, 0.0, 0.0), lcar=1.0)
+    p2 = geom.add_point((0.0, 1.0, 0.0), lcar=1.0)
+    p3 = geom.add_point((2.0, 1.0, 0.0), lcar=1.0)
+    l0 = geom.add_line(p0, p1)
+    l1 = geom.add_line(p1, p3)
+    l2 = geom.add_line(p3, p2)
+    l3 = geom.add_line(p2, p0)
+    ll0 = geom.add_line_loop((l0, l1, l2, l3))
+    rs0 = geom.add_surface(ll0)
+    geom.set_transfinite_lines([l3, l1], 3, progression=1)
+    geom.set_transfinite_lines([l2, l0], 3, progression=1)
+    geom.set_transfinite_surface(rs0)
+    geom.set_recombined_surfaces([rs0])
+
+    mesh = pygmsh.generate_mesh(geom)
+
+    assert "quad" in mesh.cells.keys()
+    ref = np.array([[0, 4, 8, 7], [7, 8, 6, 2], [4, 1, 5, 8], [8, 5, 3, 6]])
+    assert np.array_equal(ref, mesh.cells["quad"])
+
+    return mesh
+
+
+if __name__ == "__main__":
+    import meshio
+
+    meshio.write("rectangle_structured.vtk", test())