Customizable, Stackable Box v3.1

Customizable, Stackable Box for Parts Collection v3.1.

The magnet was removed, just using edge grip for cover and stacking. The stacking is fully compatible with [Customizable, Stackable Box for Parts Collection (v2.0)](https://www.thingiverse.com/thing:6173476).

See also: [Customizable Toolbox v1.0](https://www.thingiverse.com/thing:6684335)

Changes on v3.1:

* Support partition customizing up to 4 partitions (adjustable each by 1/100 of length)

Changed on v3.0:

* Thickness of all walls are reduced by default, so it is lighter and needs less PLA.
* Made the top edge separated from the side walls to make it stronger than walls.
* Added an angle at the bottom of the side walls to make it easy to detach.
* For the v2, the internal space was divided as three blocks of the almost the same size but now v3 has spaces of 1/2, 1/5, and 3/10.