Prevent parallel simplex when solving MIPs

[jajhall]

If the parallel option is set "on" by a user in an attempt to run the parallel MIP solver, don't allow this to cause a switch from kSimplexStrategyDual to kSimplexStrategyDualMulti (in HEkk::chooseSimplexStrategyThreads) otherwise the MIP solver may use parallel dual simplex - which is less robust.

A couple of un-used options have been deleted

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.94%. Comparing base (7f3f1c3) to head (dd33e6e).
⚠️ Report is 375 commits behind head on latest.

Additional details and impacted files

@@           Coverage Diff           @@
##           latest    #3042   +/-   ##
=======================================
  Coverage   72.94%   72.94%           
=======================================
  Files         426      426           
  Lines      102198   102185   -13     
  Branches    16443    16443           
=======================================
- Hits        74544    74540    -4     
+ Misses      27378    27369    -9     
  Partials      276      276           

☔ 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.

[Opt-Mucca]

Very happy with removing the unused options!

I'm assuming this call-path never affects any LP solve that's unrelated to a MIP solve? If so then I'm happy with the change!

[jajhall]

I'm assuming this call-path never affects any LP solve that's unrelated to a MIP solve? If so then I'm happy with the change!

It will if someone wants to invoke the parallel simplex solver by setting parallel="on" and simplex_strategy=kSimplexStrategyDual, but I'll document this in the release notes. However,

• there are two parallel dual simplex solvers, so it's better to force users to select the one they want, rather than only using PAMI
• the use of parallel dual simplex is niche
• it's preferable to having the MIP solver fail due to parallel simplex being used