Integrate Direct Meshing

[jmwright]

There is a direct mesh plugin which does not have any dependencies like Gmsh, and this PR is an attempt to pull that functionality into CadQuery core.

[codecov]

Codecov Report

❌ Patch coverage is 98.90110% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.88%. Comparing base (2e05819) to head (af49d4b).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
cadquery/occ_impl/assembly.py	98.90%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1953      +/-   ##
==========================================
+ Coverage   95.84%   95.88%   +0.03%     
==========================================
  Files          29       29              
  Lines        7922     8012      +90     
  Branches     1191     1210      +19     
==========================================
+ Hits         7593     7682      +89     
  Misses        192      192              
- Partials      137      138       +1     

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[shimwell]

Just chatting with @jmwright about this PR and wondering if we can make use of parallel OCCT meshing https://dev.opencascade.org/doc/refman/html/class_b_rep_mesh___incremental_mesh.html#a8048e02dc1567dcd7c846d299dd2f247