Add class module; compatible to latest OpenSCAD...

[aleung]

This PR includes some changed in my fork within two years:

1. Add class module (usage example in below)
2. Fix warnings in latest OpenSCAD version (WARNING: len() parameter could not be converted #18, warning when loading strings.scad - even when doing nothing #19)
3. Fix rod and ball distortion on small $fn (Shape difference between rod and cylinder #12?)

differed("cut", "not(cut)") {
  mirrored(x)
    for (offset = [0 : wall_thickness+wall_distance : width/2] ) {
      translated(x*offset)
        wall(sinkage(offset));
    }

  class("cut")
    hulled() {
      box([width-wall_thickness*2, 0.1, 0.01], anchor=bottom);
      translated(y*depth)
        box([width-wall_thickness*2, 0.1, height*2/3-1], anchor=bottom-y);
    }
}

[davidson16807]

Just ran on all the usual demos and it seems to work. The changes look sensible. Thanks again for all the help in keeping the project up to date with the rest of openscad! I'm hoping someone might come forward who's interested in being a project maintainer (see issue #22) so if you're at all interested then please let me know and I can reconfigure the project so that you can commit directly to master.