Assembly hierarchy - 2nd take

[adam-urbanczyk]

This PR solves nested assembly naming and enforces unique names on a single assembly level.

[adam-urbanczyk]

CC: @bernhard-42

[codecov]

Codecov Report

Merging #545 (f2c6717) into master (ac585b4) will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #545      +/-   ##
==========================================
+ Coverage   94.15%   94.30%   +0.14%     
==========================================
  Files          29       29              
  Lines        6189     6334     +145     
  Branches      655      670      +15     
==========================================
+ Hits         5827     5973     +146     
  Misses        224      224              
+ Partials      138      137       -1     

Impacted Files	Coverage Δ
cadquery/assembly.py	90.45% <100.00%> (+0.45%)	⬆️
tests/test_assembly.py	100.00% <100.00%> (ø)
tests/test_selectors.py	100.00% <0.00%> (ø)
cadquery/cq.py	89.65% <0.00%> (+0.10%)	⬆️

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[jmwright]

Looks ok to me. Thanks.

[adam-urbanczyk]

@bernhard-42 do you agree with merging this for now? Based on the discussion in #543 you have some design choices to be made that might take some time.

[bernhard-42]

@adam-urbanczyk fine for me. I do have a plan forward. Thanks for adding the hierarchies and the __classes__ thing

[adam-urbanczyk]

OK, I'll merge tomorrow.