Slack Lack Parametric Magnets

This is a remix of the Slack Lack parts which include magnets to allow parameter settings for the magnet dimensions.

Please note, the STLs included here are blanks, with no magnet insets defined. The blanks were taken from Slack Lack's step files and simply had the existing magnets extruded out.

If you have any issues with magnet alignment or positioning, please let me know. I'm still working on printing all of my parts.

Some guidelines for magnet choices, the max for dimensions is likely around 30mm length, 10mm width, and 5mm depth. Ideally you should stay under those numbers.

Use the customizer to generate ones with your given dimensions, or download everything locally and use OpenSCAD to generate the STLs. If you're on Linux or Mac, you can just run "make all" in the directory to generate all of them. Otherwise, you can use the following to generate all of them:

```
openscad -Dpart=\"OpenTab1\" magnets.scad -o generated_OpenTab1.stl
openscad -Dpart=\"OpenTabBR\" ma