dxf splines and ellipse import support

[adam-urbanczyk]

Will resolve #373

[codecov]

Codecov Report

Merging #406 into master will decrease coverage by 0.00%.
The diff coverage is 92.98%.

@@            Coverage Diff             @@
##           master     #406      +/-   ##
==========================================
- Coverage   93.56%   93.55%   -0.01%     
==========================================
  Files          19       19              
  Lines        5083     5139      +56     
  Branches      523      530       +7     
==========================================
+ Hits         4756     4808      +52     
- Misses        205      209       +4     
  Partials      122      122              

Impacted Files	Coverage Δ
cadquery/occ_impl/importers.py	87.50% <90.69%> (+1.60%)	⬆️
tests/test_importers.py	93.75% <100.00%> (+1.75%)	⬆️

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[adam-urbanczyk]

@jmwright @dcowden do you have examples of dxf files with splines/rational splines?

[jmwright]

@adam-urbanczyk I don't have any at hand, no.

[adam-urbanczyk]

I think it is ready. NB: I generated rational spline in ezdxf and imported it for the lack of a better test method. It seems to be working fine, random drawings exported from Inkscape import fine. Not all operations work well for all geometries though (filleting is picky) but I was always able to extrude the solid. Still an experimental feature but I think it is as good as it gets for now.

[jmwright]

Looks good as far as I can tell, thanks!