add the choice to not import all dxf layers

[greyltc]

this is especially useful for ignoring layers in drawings containing note text or dimensions

[jmwright]

Looks like a good to me, thanks @greyltc

@adam-urbanczyk Any objections to merging if all the tests pass?

[adam-urbanczyk]

Please don't merge yet - I'll have some comments.

[jmwright]

@adam-urbanczyk Ok

@greyltc Looks like the black lint check failed. That's a new-ish thing that's been added to CI. Let me know if you want an explanation.

[codecov]

Codecov Report

Merging #442 into master will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #442      +/-   ##
==========================================
+ Coverage   93.58%   93.67%   +0.09%     
==========================================
  Files          25       25              
  Lines        5316     5377      +61     
  Branches      554      562       +8     
==========================================
+ Hits         4975     5037      +62     
  Misses        215      215              
+ Partials      126      125       -1     

Impacted Files	Coverage Δ
cadquery/occ_impl/importers.py	88.33% <100.00%> (+0.83%)	⬆️
tests/test_importers.py	94.59% <100.00%> (+0.84%)	⬆️
tests/test_selectors.py	100.00% <0.00%> (ø)
cadquery/selectors.py	99.26% <0.00%> (+0.16%)	⬆️

---

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[adam-urbanczyk]

OK, I changed the naming slightly (I'd like to eventually add an include parameter) and excluded layers are not parsed now. Are you willing to add a test for this functionality @greyltc ? If not, I'm OK with merging as-is.

[greyltc]

Yeah, I can test!

edit: oh, you mean a proper test.
I'll give that a shot too.

[greyltc]

I think this is ready to be merged. The travis issues giving the red x there are not related to this pr.

[adam-urbanczyk]

All green - merging. Thanks @greyltc !