Added Highs::getFixedLp(HighsLp& lp)

[jajhall]

As with Gurobi, only generates the fixed LP. Users will have to pass the LP to HiGHS (losing the corresponding MIP) and then solve it.

[jajhall]

Now to add C API

[codecov]

Codecov Report

❌ Patch coverage is 99.59514% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.77%. Comparing base (dc852ec) to head (fd4b6f5).
⚠️ Report is 19 commits behind head on latest.

Files with missing lines	Patch %	Lines
check/TestMipSolver.cpp	99.04%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           latest    #2559      +/-   ##
==========================================
+ Coverage   79.71%   79.77%   +0.06%     
==========================================
  Files         346      346              
  Lines       86427    86667     +240     
==========================================
+ Hits        68891    69136     +245     
+ Misses      17536    17531       -5     

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[jajhall]

This is now ready for review @galabovaa

[jajhall]

Closes #2557

[galabovaa]

Looks good to me! Just one conflict but it seems that we should just accept both tests