To do
=====

[X] Actually use newly added gerber samples in test suite
[X] Make Gerber parser error out if no unit is set anywhere
[ ] Add test case for board outline / bounds with arcs (e.g. circle made up of four arcs, each with center line along
    x/y axis)
[ ] Add "find outline" method
[X] Refactor layer identification logic, automatically detect Allegro NCPARAM files
[X] Add idempotence test: When reading, then reserializing the output of an earlier invocation of gerbonara, the output
    should not change. That is, in f1 --gn-> f2 --gn-> f3, it should be f2 == f3 but not necessarily f1 == f2.
[X] Handle upverter output correctly: Upverter puts drils in a file called "design_export.xln" that actually contains
    Gerber, not Excellon
[X] Add standard comment/attribute support for Gerber and Excellon
[ ] Add attribute support to gerber output
[X] Add file/lineno info to all warnings and syntax errors
[X] Make sure we handle arcs with co-inciding start/end points correctly (G74: no arc, G75: full circle)
[ ] Add allegro drill test files with different zero suppression settings
[ ] Add pcb-rnd to layer matching
[ ] Add librepcb to layer matching
[ ] On altium exports with multiple mech layers, use lowest-numbered one as board outline and raise a warning.
[ ] Assign layer rules based on allegro metadata instead of filenames for allegro files
[ ] Add more IPC-356 test files from github
[X] Add IPC netlist support to LayerStack
[ ] It seems the excellon generator never generates M16 (drill up) commands, only M15 (drill down) commands during
    routing
[ ] Add standalone excellon SVG export test
[ ] In image difference tests, detect empty images.
[ ] Merge subsequent paths in gerbv svgs for less bad rendering performance
[ ] Add integrated zip handling to layerstack
[ ] Add GraphicObject.as(unit) method
[ ] Add methods to graphic_object Line, Arc, Flash to convert between gerber and excellon representations.
[ ] Add to_primitives to all *File classes
[ ] Add region cut-in API
[ ] Add radius- instead of center-based method of creating Arcs
[ ] Add warning when interpolating aperture that is not either a circle or a rectangle. 
[ ] Maybe have Line and Arc just use a width instead of an aperture after all, and roll plating of excellon tool into
    graphic object subclass.
[ ] Figure out whether to drop rectangular holes or whether to support them in polygon apertures as well.
[ ] Add "number of parameters" property to ApertureMacro
[ ] Aperture macro outline: Warn if first and last point are not the same.
[ ] Make sure incremental mode actually works for gerber import
[ ] Add text rendering function