More consistent handling of BoundaryBox tolerance

[marcus7070]

I was working on some doc strings for another PR and noticed some inconsistencies in how the tolerances on bounding boxes were handled. This PR changes BoundBox.add tolerance to default to TOL, just like the BoundBox._fromTopoDS method. Also Shape.centerOfBoundBox previously had another different default tolerance that wasn't propagated anyway.

[codecov]

Codecov Report

Merging #490 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #490   +/-   ##
=======================================
  Coverage   93.65%   93.65%           
=======================================
  Files          30       30           
  Lines        5880     5881    +1     
  Branches      626      626           
=======================================
+ Hits         5507     5508    +1     
  Misses        234      234           
  Partials      139      139           

Impacted Files	Coverage Δ
cadquery/occ_impl/geom.py	88.96% <ø> (+0.02%)	⬆️
cadquery/occ_impl/shapes.py	90.94% <ø> (ø)

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[jmwright]

Thanks for fixing this @marcus7070

+1 to merge after you've had a chance to take a look @adam-urbanczyk

[adam-urbanczyk]

Thanks @marcus7070 , looks good!