DXF multilayer support (#1267)

* Test DXF export with text

* DXF multilayer support

Also supports:

* setting units
* setting color by layer
* setting line type by layer

* Apply transformation to bspline curve on DXF export

* Typing improvements

* Improve test coverage

---------

Co-authored-by: Lorenz Neureuter <[email protected]>

Author: sethfischer

cadquery/occ_impl/exporters/__init__.py

@@ -15,7 +15,7 @@
 from .json import JsonMesh
 from .amf import AmfWriter
 from .threemf import ThreeMFWriter
-from .dxf import exportDXF
+from .dxf import exportDXF, DxfDocument
 from .vtk import exportVTP
 from .utils import toCompound