Fix gerber-to-excellon conversion

gerbonara/layers.py

@@ -518,7 +518,7 @@ class LayerStack:
             return self.bounding_box(unit=unit, default=default)
 
     def merge_drill_layers(self):
-        target = ExcellonFile(comments='Drill files merged by gerbonara')
+        target = ExcellonFile(comments=['Drill files merged by gerbonara'])
 
         for layer in self.drill_layers:
             if isinstance(layer, GerberFile):

gerbonara/rs274x.py

@@ -63,14 +63,14 @@ class GerberFile(CamFile):
         new_objs = []
         new_tools = {}
         for obj in self.objects:
-            if not isinstance(obj, go.Line) or isinstance(obj, go.Arc) or isinstance(obj, go.Flash) or \
+            if (not isinstance(obj, go.Line) and isinstance(obj, go.Arc) and isinstance(obj, go.Flash)) or \
                 not isinstance(obj.aperture, apertures.CircleAperture):
-                raise ValueError(f'Cannot convert {type(obj)} to excellon!')
+                raise ValueError(f'Cannot convert {obj} to excellon!')
 
             if not (new_tool := new_tools.get(id(obj.aperture))):
                 # TODO plating?
-                new_tool = new_tools[id(obj.aperture)] = apertures.ExcellonTool(obj.aperture.diameter, plated=plated)
+                new_tool = new_tools[id(obj.aperture)] = apertures.ExcellonTool(obj.aperture.diameter, plated=plated, unit=obj.aperture.unit)
-            new_obj = dataclasses.replace(obj, aperture=new_tool)
+            new_objs.append(dataclasses.replace(obj, aperture=new_tool))
             
         return ExcellonFile(objects=new_objs, comments=self.comments)