Adding Problem::set_matrix_structure_only

[andrsd]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.92%. Comparing base (8e56e63) to head (1960151).
⚠️ Report is 9 commits behind head on 0.16.x.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/Problem.cpp	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           0.16.x    #1069      +/-   ##
==========================================
- Coverage   78.94%   78.92%   -0.03%     
==========================================
  Files         164      164              
  Lines       10944    10947       +3     
==========================================
  Hits         8640     8640              
- Misses       2304     2307       +3     

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