Fix incompatible pointer type assignment error in implicit_surface

Actual output:

error: assignment to 'PyObject *' {aka 'struct _object *'} from incompatible pointer type 'struct __pyx_obj_4sage_4plot_6plot3d_16implicit_surface_VertexInfo *' [-Wincompatible-pointer-types

src/sage/plot/plot3d/implicit_surface.pyx

@@ -522,7 +522,7 @@ cdef class MarchingCubesTriangles(MarchingCubes):
                     if not(self.color_function is None):
                         self.apply_color_func(&v.color, self.color_function,
                                               self.colormap, v)
-                    y_vertices[y,z] = v
+                    y_vertices[y,z] = <object>v
                 else:
                     y_vertices[y,z] = None
 
@@ -556,7 +556,7 @@ cdef class MarchingCubesTriangles(MarchingCubes):
                     if not(self.color_function is None):
                         self.apply_color_func(&v.color, self.color_function,
                                               self.colormap, v)
-                    z_vertices[y,z] = v
+                    z_vertices[y,z] = <object>v
                 else:
                     z_vertices[y,z] = None
 
@@ -631,7 +631,7 @@ cdef class MarchingCubesTriangles(MarchingCubes):
                     if not(self.color_function is None):
                         self.apply_color_func(&v.color, self.color_function,
                                               self.colormap, v)
-                    x_vertices[y,z] = v
+                    x_vertices[y,z] = <object>v
                 else:
                     x_vertices[y,z] = None