Enhancement: Extract named constructors for ReadOnlyVariables

[localheinz]

This PR

• extracts named constructors for ReadOnlyVariables

[codecov]

Codecov Report

Merging #20 into master will increase coverage by 0.17%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master      #20      +/-   ##
============================================
+ Coverage     92.72%   92.89%   +0.17%     
- Complexity       86       88       +2     
============================================
  Files            11       11              
  Lines           165      169       +4     
============================================
+ Hits            153      157       +4     
  Misses           12       12              

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