Order path separator

[Opt-Mucca]

This PR sorts rows via their fractionalActivity in HighsPathSeparator when considering which row to add to the aggregation.
High level overview: The separator tries to find a continuous column to aggregate out, and to do so finds an additional row that also contains that continuous column. Which row to use is non-obvious.

The old method:

• No sorting. Given the array of rows, each time you want to select a new row, scan through the array starting at a random point.

The new method:

• Sort rows via their fractional activities. Scan through them in order each time.

Pros:

• More informed ordering. Should prioritise rows that can generate good cMIR cuts.
• Randomly selecting a starting point to scan through the array already kind of enforced a priority. That is, array[i-1] is checked before array[i] in all cases aside from starting the scan at i.

Cons:

• Losing some inherent diversity that a random algorithm provides. It's now more likely that we'd aggregate similar rows each time, which could lead to many duplicate cuts.

This should only be merged after doing a performance run.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.25%. Comparing base (e4632db) to head (48be3da).
⚠️ Report is 68 commits behind head on latest.

Additional details and impacted files

@@            Coverage Diff             @@
##           latest    #2674      +/-   ##
==========================================
+ Coverage   81.20%   81.25%   +0.04%     
==========================================
  Files         349      349              
  Lines       85465    85801     +336     
==========================================
+ Hits        69406    69721     +315     
- Misses      16059    16080      +21     

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[fwesselm]

Looks good, @Opt-Mucca. I will try to qualify as soon as possible.

[fwesselm]

Could you have

double absval = std::abs(lp.a_matrix_.value_[i]);

here and use this in the following two lines (instead of having the std::abs twice)?

[fwesselm]

This vector does not have to be re-initialized (with zeros), correct?

[fwesselm]

There is an utility

inline T fractionality(T input, T* intval = nullptr)

in HighsLpUtils.h that you could maybe use here.