Enhance IIS

[jajhall]

The existing IIS facility has been refactored and is much more robust. Rather than have HighsOption::iis_strategy be one from an enum of "strategy scenarios" it is now a bit map

• 0 => "light strategy", which is always performed when Highs::getIis is called
• 1 => From dual ray, which is currently unavailable
• 2 => From the whole LP (solving an elasticity LP repeatedly (fixing positive elastic variables at zero) until no more elastic variables are positive, and using the fixed elastic variables to determine a set of infeasible rows, for which there is a corresponding set of columns with nonzeros in those rows that form an infeasibility set (IS)
• 4 => Attempt to reduce the IS to an ISS
• 8 => Prioritize low numbers of columns (rather than low numbers of rows) when reducing the IS

Hence, by just setting the 2-bit, an IS is formed reliably, and at not great expense (for an LP)

Enhancements to be added in due course

• If time-out occurs when reducing the IS, return the current IS
• Apply the strategies above to a MIP
• Use the 16-bit to attempt to find an IIS of a MIP relaxation, rather than the MIP itself. The latter calculation is very expensive since it requires the solution of multiple MIPs.

Clearly if the relaxation of a MIP is infeasible, an IIS is found very much more cheaply. Indeed, it makes sense to test whether the relaxation is infeasible before attempting to find even an IS of a MIP.

[codecov]

Codecov Report

❌ Patch coverage is 93.01802% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.22%. Comparing base (e8e644f) to head (5f994bd).
⚠️ Report is 39 commits behind head on latest.

Files with missing lines	Patch %	Lines
highs/lp_data/HighsInterface.cpp	89.33%	16 Missing ⚠️
highs/lp_data/HighsIis.cpp	91.58%	9 Missing ⚠️
check/TestIis.cpp	95.83%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           latest    #2644   +/-   ##
=======================================
  Coverage   81.22%   81.22%           
=======================================
  Files         349      349           
  Lines       85780    85780           
=======================================
  Hits        69676    69676           
  Misses      16104    16104           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[galabovaa]

LGTM!