Use Intersection = True in shell

[adam-urbanczyk]

Related to google groups discussion. After this modification shelling takes into account intersection between non-adjacent faces:

[codecov]

Codecov Report

Merging #411 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #411   +/-   ##
=======================================
  Coverage   93.55%   93.56%           
=======================================
  Files          19       19           
  Lines        5139     5142    +3     
  Branches      530      530           
=======================================
+ Hits         4808     4811    +3     
  Misses        209      209           
  Partials      122      122           

Impacted Files	Coverage Δ
cadquery/occ_impl/shapes.py	90.67% <ø> (ø)
tests/test_cadquery.py	98.97% <100.00%> (+<0.01%)	⬆️

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[jmwright]

Looks good, thanks. Merging