Add option for hexagonal holes for M4 nuts

[rlew-is]

Added the option to add hexagonal holes to the base plates. This is setup so that they can accept M4 nuts.
I have found these to be significantly cheaper than installing hundreds of magnets on both plates and bins. Bonus being no need to orientate N/S poles.
It uses the existing ribbed_cylinder function, but with a quantity of 6 instead of 8. After some testing I have set the inner and outer radii to allow a firm thumb push without the magnets being able to remove them.
I have updated the python tests to include the new hex holes. There is also an OpenSCAD conditional to prevent hex holes and crush ribs being used together.

Would love to have others test and see if it works as intended.
This is my first PR so let me know if anything needs doing differently.