deprecate bezier_surface and function_grapher

src/bezier_surface.scad

@@ -13,6 +13,7 @@ use <matrix/m_transpose.scad>;
 
 function bezier_surface(t_step, ctrl_pts) =
     let(
+        _ = echo("`bezier_surface` is deprecated since 3.1. Use `sf_splines` instead."),
         leng_ctrl_pts = len(ctrl_pts),
         pts =  [
         for(i = 0; i < leng_ctrl_pts; i = i + 1)

src/function_grapher.scad

@@ -14,6 +14,7 @@ use <path_extrude.scad>;
 use <shape_circle.scad>;
 
 module function_grapher(points, thickness = 1, style = "FACES") {
+    echo("`function_grapher` is deprecated since 3.1. Use `sf_thicken` instead."),
 
     rows = len(points);
     columns = len(points[0]);