Custom paint brush shelves for modular paint stack

This allows for the creation of custom shelves for paint brushes that work with the Modular Paint Stack (MPS). The print profile includes a brush diameter test piece you can print this out first and then test the sizes of your brushes. Since many are tapered, this is a good way to figure out what size allows them all to stick out roughly the same amount. There are two different ways to customize: shelf_brushes (Simple shelf for brushes) - This lets you pick up to four rows where each contains the specified number of holes at a given size. This can make nice geometric patterns and works well if your brushes all have the same size handles. To make working with the Maker World SCAD customizer easy, there is a pair of values for each row: hole_diameter and hole_count. Set the hole diameter to the size that worked for you with the test piece and set the count to the number of holes of that size in that row. If you want less than four rows, set the hole_count to 0 to skip that row. shelf_bru