Z axis printability enhancements

There are several places inside the z-axis where there are overhangs that are not bridges. This has not caused a problem in actual prints, but it does not follow the design of the rest of the microscope to be careful with bridging and overhangs.

There are three reasons for the overhangs and for bridges which are not straight, which are addressed by this MR:

• The hole for the hex key access cuts through the top of the hole for the bottom moving lever of the z-flexure, so closing the top of the hole is not a complete bridge. This is addressed by making the angle of the top of that hole be 25degrees to horizontal, which is printable. 4f06cc09
• The hole for the hex key access is at a slight rising angle and is defined as a cylinder. This is addressed by defining it as a printable_horizontal_hole 4c8b2321
• The roof above the lever to the z-axis actuator is a slightly sloping curve. This is as a result of hull() of parts with cylindrical upper section, and then also a minkowski() with a vertical cylinder. These are addressed by using cylinders with a square top ( acc2f594 ), and minkowski() with a cube ( 5f354edb )

This should not make a functional difference to the microscope operation, and it importantly leaves the footprint on the build plate with the curved corners which are there to help to prevent lifting.