DXF export (2D) and exporters cleanup

[adam-urbanczyk]

Will resolve #352 , #340, #212, #334 and #360 .

This adds dxf export. Moreover I implemented a new export function and marked exportShape as deprecated:

def export(
    w: Union[Shape, Workplane],
    fname: str,
    exportType: Optional[ExportLiterals] = None,
    tolerance: float = 0.1,
): ...

Now exporting is more user friendly: exporters.export(obj,'out.step')

[codecov]

Codecov Report

Merging #415 into master will increase coverage by 0.02%.
The diff coverage is 95.75%.

@@            Coverage Diff             @@
##           master     #415      +/-   ##
==========================================
+ Coverage   93.56%   93.59%   +0.02%     
==========================================
  Files          19       25       +6     
  Lines        5147     5291     +144     
  Branches      530      548      +18     
==========================================
+ Hits         4816     4952     +136     
- Misses        209      215       +6     
- Partials      122      124       +2     

Impacted Files	Coverage Δ
cadquery/occ_impl/exporters/__init__.py	92.63% <92.63%> (ø)
cadquery/occ_impl/exporters/dxf.py	92.85% <92.85%> (ø)
cadquery/__init__.py	100.00% <100.00%> (ø)
cadquery/cq.py	88.06% <100.00%> (+0.13%)	⬆️
cadquery/occ_impl/exporters/amf.py	100.00% <100.00%> (ø)
cadquery/occ_impl/exporters/json.py	100.00% <100.00%> (ø)
cadquery/occ_impl/exporters/svg.py	95.19% <100.00%> (ø)
cadquery/occ_impl/exporters/utils.py	100.00% <100.00%> (ø)
cadquery/occ_impl/jupyter_tools.py	100.00% <100.00%> (ø)
cadquery/occ_impl/shapes.py	90.69% <100.00%> (+0.01%)	⬆️
... and 10 more

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[jmwright]

There's a lot here so I could have missed some things, but I think it looks good. It's nice to have the exporters broken out and organized rather than all dumped in one (or very few) files. Thanks!