Corrected teardrop_plus() shape to be an accurate compensation for slicer

staircasing and added a plus option to tearslot(), etc.

Added horiholes.scad to depict staircase holes.

tests/horiholes.scad

@@ -0,0 +1,86 @@
+//
+// NopSCADlib Copyright Chris Palmer 2020
+// nop.head@gmail.com
+// hydraraptor.blogspot.com
+//
+// This file is part of NopSCADlib.
+//
+// NopSCADlib is free software: you can redistribute it and/or modify it under the terms of the
+// GNU General Public License as published by the Free Software Foundation, either version 3 of
+// the License, or (at your option) any later version.
+//
+// NopSCADlib is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
+// without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+// See the GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License along with NopSCADlib.
+// If not, see <https://www.gnu.org/licenses/>.
+//
+$layer_height = 0.25;
+include <../utils/core/core.scad>
+use <../utils/horiholes.scad>
+
+show_disc = true;
+thickness = 3;
+length = 50;
+height = 20;
+overlap_x = 15;
+overlap_y = 5;
+
+module hole_positions() {
+    for($i = [0 : 4], $z = 5 + $i * layer_height / 5, $r = 3)
+        translate([5 + $i * 10, $z])
+            children();
+
+    for($i = [0 : 4], $z = 15 + $i * layer_height / 5, $r = 0.5 + $i / 2)
+        translate([5 + $i * 10, $z])
+            children();
+}
+
+module horiholes_stl(t = thickness) {
+    rotate([90, 0, 0])
+        difference() {
+            linear_extrude(t, center = true) {
+                difference() {
+                    square([length, height]);
+
+                    hole_positions()
+                        horihole($r, $z);
+                }
+            }
+        }
+    if(t == thickness)
+        translate([length / 2, 0])
+            rounded_rectangle([length + 2 * overlap_x, thickness + 2 * overlap_y, 2], 5);
+}
+
+module horiholes() {
+    stl_colour(pp1_colour)
+        rotate([-90, 0, 0])
+            horiholes_stl(eps);
+
+    if(show_disc)
+        hole_positions()
+            color(silver)
+                cylinder(r = $r, h = eps, center = true, $fn = 360);
+
+    hole_positions()
+        color("red")
+            linear_extrude(2 * eps, center = true)
+                //offset(0.01, $fn = 360)
+                    intersection() {
+                        difference() {
+                            square(8, center = true);
+
+                            horihole($r, $z);
+                        }
+
+                        circle($r, $fn = 360);
+                    }
+}
+
+if($preview)
+    rotate(is_undef($bom) ? 0 : [70, 0, 315])
+        horiholes();
+else
+    horiholes_stl();