Fix backwards compatibility nut_sides arg

* nut_sides must be last argument
* default value = 0. With 120 as default, we cannot set nut_sides to 120
  while simultaneously setting fn to something different from 120.

docs/img_prep/nut_sides.scad

@@ -0,0 +1,15 @@
+/*
+Demo nut_sides argument of nut() module.
+*/
+
+use <threadlib/threadlib.scad>
+include <../../THREAD_TABLE.scad>
+
+type = "M12x0.5";
+turns = 7;
+Douter = 16;
+higbee_arc = 45;
+fn = 16;
+nut_sides = 6;
+
+nut(type, turns, Douter, higbee_arc, fn, THREAD_TABLE, nut_sides);