laserscad v0.2.1

This is a bugfix release for v0.2.0.

• Added more checks and messages:
  • make will terminate...
    • if laserscad.scad can't be found
    • if no lparts are defined
    • if duplicate lpart ids are used
  • lslice and lengrave now check all arguments for invalid values
  • more elaborate checks of lpart arguments
  • when calling make, any errors, warnings and echo statements unrelated to laserscad are printed
• SVG output by make engrave is now configurable (see readme):
  • unit can be specified
  • stroke width can be specified or disabled to fix problems with small/detailed shapes
• LGPLv3 license headers added to all files