Use expression grammar for assy selectors

[adam-urbanczyk]

Will resolve #703

[codecov]

Codecov Report

Merging #704 (179b964) into master (6d3c9e5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #704   +/-   ##
=======================================
  Coverage   94.42%   94.42%           
=======================================
  Files          31       31           
  Lines        6909     6911    +2     
  Branches      740      740           
=======================================
+ Hits         6524     6526    +2     
  Misses        251      251           
  Partials      134      134           

Impacted Files	Coverage Δ
cadquery/assembly.py	90.54% <100.00%> (ø)
tests/test_assembly.py	100.00% <100.00%> (ø)

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[jmwright]

Thanks @adam-urbanczyk