Add extra bore depth to nut traps, add chamfer option to nut traps

1. To aid pinching up nuts into a nut trap with a M3x10mm screw, the bore in nut traps needs to allow at least 10mm deep from the top surface. This was noted for the illumination mount in #198 (closed), but can be accommodated for all of the nut traps.

• illumination mount - 3mm extra depth (11mm clearance from top)
• z-axis spacers - 2.5mm extra depth (10.5mm clearance from top), to avoid breaking through
• x-y stage top - 3mm extra depth (11mm clearance from top)
• mounting lugs in microscope stand - 2.2mm extra depth (10.2mm clearance from top), to avoid printing problems when the hole penetrates the sloping lower surface of the lug.
• nut trap test - 3mm extra depth now bores right through (the block is only 10mm high)

2. Many nut traps penetrate curved perimeters. This gives curved bridges which sag, adding to assembly problems like #292 (closed). A chamfer option adds the ability to make a 30degree angle to the top of the slot, making the first bridges straight with a printable slope out to the curved surface.

• illumination mount - used here
• z-axis spacers - used here
• x-y stage top - not used as the surfaces are flat
• mounting lugs in microscope stand - used here
• nut trap test - not used as the surfaces are flat

closes: #198 (closed)