Changes

internal: can set bevel to true and get non-garbage result
   bevel is always set by thread depth
   acme takes tpi
   square threads are at angle 0
   added generic_threaded_{rod,nut}
   eliminated metric_trapezoidal_*
   cleaned up matrices some in generic_threaded_rod
   threaded_rod can produce spec-true ISO/UTS profile with a triplet input for the diameter.
   Added bevel1 and bevel2 to all modules.  Made default uniformly false for every case instead of
       sometimes true, sometimes false
   Profiles that go over zero are not clipped, and bevels are based on actual profile top, not nominal
   When bevel is given to nuts it bevels the outside of the nut by thread depth
   higbee looks best with quincunx, but it's more expensive.  Select quincunx when higbee is used, min_edge otherwise
   Current code uses difference to remove excess length in the rod.  This gives faster renders at the cost
      of more complex code and green top/bottom surfaces.
   Changed slop to 4 * $slop.  I got good results printing with $slop=0.05 with this setting.
   Removed excess threads from vnf (affects case where starts>1), don't force threads to be even

   Fixed higbee in spiral_sweep for properly centered scaling and for staying on the internal/external base of threads
   Fixed bug in spiral_sweep where two segments were missing if higbee is zero

threading.scad

@@ -220,6 +220,8 @@ module trapezoidal_threaded_rod(
                         affine3d_translate([0,pdepth,0])
                 ) each apply(mat3, prof3d)
             ])
+
+            
         ) [
             [0, 0, -l/2-pitch],
             each apply(mat1*mat2, prof),