interlocking grid

Interlocking grid system initially made for holding small petri dishes in position on a tray. Option for a floor means that racks can be made for bottles etc.
Check https://github.com/davidelliott/parametric-lab for the latest version or to contribute your improvements.
If you use the customiser note the following:
cells_x must equal cells_y
only build grid or walls (grid=0/1 , walls=0/1). If you build both together it may crash.