Improvements to Gridfinity Base

[maihde]

This PR provides multiple improvements, three are very straight forward:

• allow setting height to zero to disable creation of walls (i.e. you only want the grid base).
• set the openings to have a radius that is consistent with the Gridfinity specification and more asthetically pleasing in my opinion
• when cut_pads is turned on, also cut magnet holes by default. I could see an argument to add a boolean cut_pads_mag which is off by default but I refrained to keep the command-line more concise.

The next is intended to make it easy to use boxes.py to build grids that drop into existing areas with a fixed size like drawers. Instead of providing the number of grids in the x and y direction you can instead specify the full size (in mm) in the sx and xy similar to (https://laserbase.makershop.ch/).

I made a few design decisions here:

• the default behavior is consistent with the existing command-line for boxes.py, so anyone creating Gridfinity trays already won't see a change
• if x, y, sx, sy are all provided then the minimum size of the box is specified by x and y(in other words, you are always guarantedxbyygrids). Ifsxandsy` are larger than the mininum size, then the grid is centered in the larger base.
• if x and y are set to zero then the maximum number of grids is produced to fit within the provided sx and sy

The next change allows creating grids that are larger than a single work piece. This is similar to how https://laserbase.makershop.ch/Split works (but better). In the makershop version, splits are made arbitrarily. In my implementation, the splits are made down the center between grids and joined with a dovetail joint. A future commit will allow turning the dovetail off and using a straight edge.

In addition, when run in split mode it will also automatically split the box walls and base. I'd really like the box base to use a deeper dovetail, but couldn't figure out how to change dovetail settings mid-render. Help would be appreciated here. In addition, when running in split mode it won't generate a lid.