OpenSCAD model for Case for PinePhone

based on [beattie's earlier model](https://github.com/beattie/PinePhone-Case).

WIP phone case for the PinePhone, and associated models.

files:
- dimensions.scad: dimensions used to build phone module
- bits.scad:       modules for the bumps on the phone to make holes in the case
- fillet.scad:     form a rounded bump around the screen opening
- phone.scad:      a module to make a model of the phone use to carve out a hole for the case
- case.scad:       make a model for the case