Various changes to HiPO

[filikat]

• The structure and values of normal equations or augmented system have been moved to a separate class KktMatrix. Some auxiliary functions used for the factorisation phase have been moved to auxiliary. This allows to use different LinearSolver objects to solve the linear systems, in preparation to the up-looking factorisation.
• HiPO now computes upper and lower bounds for the number of nonzeros in the normal equations matrix. If the augmented system is much sparser than the lower bound of normal equations, the former is selected automatically. If the augmented system is much dense than the upper bound of normal equations, the latter is selected automatically. This approach avoids applying metis or amd to matrices that are very unlikely to be preferred, saving some time in the analyse phase for some problems. This approach is much more effective than guessing based on the presence of dense columns.
• The HiPO interface to HiGHS logging has been simplified.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.30%. Comparing base (073b504) to head (0688910).
⚠️ Report is 9 commits behind head on latest.

Additional details and impacted files

@@           Coverage Diff           @@
##           latest    #2978   +/-   ##
=======================================
  Coverage   81.29%   81.30%           
=======================================
  Files         358      359    +1     
  Lines       88753    88767   +14     
=======================================
+ Hits        72151    72170   +19     
+ Misses      16602    16597    -5     

☔ 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.