Gridfinity Selective Bases

A contribution to the gridfinity-rebuilt-openscad project adding a base_locations option that lets users skip rendering bases in the center of a grid bin, keeping only corners or edges. This reduces print time and saves plastic for larger bins.

Inspired by a Reddit thread about omitting inner base squares. Two modes are offered: corners only and edges only. Without supports, bridging ability limits how large you can go with corners-only (3x3 worked fine on a Prusa MK3, but larger sizes needed supports).