Add back add_string_variable

[magnusuMET]

Fixes #154

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.43%. Comparing base (1e4d7d6) to head (3d281b1).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #162      +/-   ##
==========================================
+ Coverage   73.34%   73.43%   +0.09%     
==========================================
  Files          14       14              
  Lines        2202     2210       +8     
==========================================
+ Hits         1615     1623       +8     
  Misses        587      587              

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