Check 2181: pass all std::string by reference

[jajhall]

Ensures that all std::string are passed by reference, plus a couple of other cases where std::vector is passed by value. Finally handles #2181! Sorry @MaximSmolskiy

[codecov]

Codecov Report

❌ Patch coverage is 52.63158% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.27%. Comparing base (998b217) to head (48487c6).
⚠️ Report is 19 commits behind head on latest.

Files with missing lines	Patch %	Lines
highs/io/HighsIO.cpp	0.00%	3 Missing ⚠️
highs/util/HighsUtils.cpp	40.00%	3 Missing ⚠️
highs/lp_data/HighsDebug.cpp	0.00%	2 Missing ⚠️
highs/lp_data/HighsIis.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           latest    #2833      +/-   ##
==========================================
+ Coverage   80.24%   80.27%   +0.02%     
==========================================
  Files         348      348              
  Lines       86193    86212      +19     
==========================================
+ Hits        69169    69210      +41     
+ Misses      17024    17002      -22     

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