Remove continuous singletons from double-sided rows

[Opt-Mucca]

General idea: We currently have no explicit reduction for a column s, with 0 cost, and that only appears in a single double-sided row b_0 <= a^Tx + cs <= b_1. Dual fixing removes all cases where b_0 == -kHighsInf or b_1 == kHighsInf, but the column is left alone if b_0 and b_1 are finite. Removing the column may allow additional reductions to be made.

Motivation: I was playing around with an instance, trying to figure out why HiGHS wasn't making a reduction, and thought this would help (spoiler: it did not), but while doing so I stumbled upon an old TODO in the code with the exact suggestion.

@fwesselm Three major questions:

• Do you think this is worth adding? I haven't performance tested this.
• If so should this be kept behind a MIP-only flag?
• I'm currently skipping the case where s has infinite bounds, but in theory we could handle that here. The issue is I'm not sure what to do with infinite values in postsolve.

Edit: Need to look into the failing tests (they're passing locally.....). Windows does not like this change at all.

[codecov]

Codecov Report

❌ Patch coverage is 84.33735% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.32%. Comparing base (9d3c86f) to head (9cce723).
⚠️ Report is 22 commits behind head on latest.

Files with missing lines	Patch %	Lines
highs/presolve/HighsPostsolveStack.cpp	78.37%	8 Missing ⚠️
highs/presolve/HighsPostsolveStack.h	70.58%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           latest    #2962      +/-   ##
==========================================
+ Coverage   81.28%   81.32%   +0.04%     
==========================================
  Files         358      358              
  Lines       88720    88724       +4     
==========================================
+ Hits        72119    72159      +40     
+ Misses      16601    16565      -36     

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[fwesselm]

This is great, @Opt-Mucca! I would like to experiment with this a llittle more. Also, there is HPresolve::removeSlacks which handles a "similar" case. If you are OK with it, I would make some more changes (on top of your change).

[Opt-Mucca]

@fwesselm I was worried that I was missing something, so please make any changes you want.