Understand why presolve cannot be used for AC calculation

[jajhall]

So, if the LP for the AC calculation is presolved, the solution obtained may well have integer variables at bounds.

This is not just a "bad" interior point, it can lead to incorrect deductions about the feasible values of integer variables, and incorrect optimal solutions of the MIP being claimed. This was behind the errors observed with egout.

All that's come out of this is that when no basis exists, a basis is only constructed from an existing solution when it might be useful - ie when the problem is not a MIP, or a MIP relaxation is being solved.

[jajhall]

This closes #2501

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.73%. Comparing base (cfd986d) to head (1058480).
⚠️ Report is 98 commits behind head on latest.

Additional details and impacted files

@@            Coverage Diff             @@
##           latest    #2507      +/-   ##
==========================================
- Coverage   79.73%   79.73%   -0.01%     
==========================================
  Files         346      346              
  Lines       85976    85982       +6     
==========================================
+ Hits        68553    68555       +2     
- Misses      17423    17427       +4     

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