Add STEP Import for Assemblies

[jmwright]

The goal is to make it possible to round-trip assemblies to and from STEP without loss of data. This data can include:

• Part location
• Part color
• Sub-assembly location
• Sub-assembly colors
• Face labels
• Face colors

[codecov]

Codecov Report

Attention: Patch coverage is 85.18519% with 12 lines in your changes missing coverage. Please review.

Project coverage is 95.42%. Comparing base (8c68faa) to head (687bc2b).

Files with missing lines	Patch %	Lines
cadquery/occ_impl/importers/assembly.py	82.60%	7 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1779      +/-   ##
==========================================
- Coverage   95.54%   95.42%   -0.12%     
==========================================
  Files          28       29       +1     
  Lines        7319     7396      +77     
  Branches     1102     1114      +12     
==========================================
+ Hits         6993     7058      +65     
- Misses        195      202       +7     
- Partials      131      136       +5     

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[jmwright]

@adam-urbanczyk In 4045c58 you set the cadquery.Assembly.importStep method up to be a class method, and you construct assy before calling the lower-level method. However, since the Assembly.name property is private and can only be set during instantiation, this creates an issue for me. I need to be able to set the top-level assembly name based on the name set in the STEP file, which requires me to set the name property after instantiation.

We need to decide on the proper way to fix this.

[adam-urbanczyk]

Hey, I do not understand the issue. You can always do this:

assy = Assembly()
assy.name = '123'

Do you mean that you need to modify AssemblyProtocol to satisify mypy? That also does not sound like an issue to me.

[jmwright]

Correct, mypy complains. I can make that change to make name a public property if you don't see an issue with doing so.