Added ability to convert linear RGB tuple to sRGB

[jmwright]

When completed, this will address #1411

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 95.59%. Comparing base (1b2c7fd) to head (774f5cd).

Files with missing lines	Patch %	Lines
cadquery/occ_impl/assembly.py	25.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1839      +/-   ##
==========================================
- Coverage   95.66%   95.59%   -0.08%     
==========================================
  Files          28       28              
  Lines        7431     7438       +7     
  Branches     1122     1122              
==========================================
+ Hits         7109     7110       +1     
- Misses        193      199       +6     
  Partials      129      129              

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