Added metadata handling to Assembly.add method

[jmwright]

I am using the metadata Dictionary to create exploded assembly views by including a relative translation with each assembly component. The metadata works fine when adding Workplanes, but when sub-assemblies are added the metadata is not preserved. This PR adds a new Assembly.add() overload, along with the relevant code to save the metadata.

Closes #1321

[codecov]

Codecov Report

Merging #1327 (3e2d1e1) into master (33bf83e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 3e2d1e1 differs from pull request most recent head 4771b8b. Consider uploading reports for the commit 4771b8b to get more accurate results

@@           Coverage Diff           @@
##           master    #1327   +/-   ##
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  Coverage   94.29%   94.29%           
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  Files          26       26           
  Lines        5575     5576    +1     
  Branches      950      950           
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+ Hits         5257     5258    +1     
  Misses        187      187           
  Partials      131      131           

Impacted Files	Coverage Δ
cadquery/assembly.py	96.61% <100.00%> (+0.01%)	⬆️

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[lorenzncode]

I've updated the test.

add will override existing metadata if any. I suppose if override is not desired the metadata dicts can be merged manually before calling add.

[adam-urbanczyk]

Thanks @jmwright