Metric screws as alternative to User defined

[pleite]

This is an implementation of screw_hole instead of the current user defined holes, this enforces threading to be tested on the mounting points for openGrid.

[AndyLevesque]

It appears that several of the head options are not compatible. They either clip into the tile or sink the majority of the way into the lite tile. Should we remove the head options for now and retain just the screw names?

Also, should we change 'None' to 'Custom' to communicate how the manual dimensions kick in?

[ejwill]

@AndyLevesque, depending on the head style larger diameter screws will clip the board or not. If you chose a larger screw then have head type as none it will be fine. So what do we want to do about that? We could use an assert to allow combos that "work". I am working on a PR that simplifies some of this and adds back custom options.

If the screw option/name is none the board has screw holes, is that right? Seems like if someone selects none there should be no screw holes.