QP support for MOSEK, COPT, MindOpt

[fneum]

MOSEK:

• The solsta.dual_infeas_cer should be interpreted as "infeasible_or_unbounded"
• An empty constraint list can lead to an AttributeError when reading out duals. This is handled like in the Gurobi interface.

COPT

• For reading out the solver status, need to handle the case where model.type == "QP"
• An empty constraint list can lead to an AttributeError when reading out duals. This is handled like in the Gurobi interface.
• The check in the test test_quadratic_model for the y variable was too restrictive. 10 is the lowest value y has to take in the cost-optimum, but increasing this value (which was what COPT did) yields the same objective value. Degeneracy.

MindOpt:

• According to the developers, MindOpt does not yet support reading in QPs via LP file, only via MPS file.
• An empty constraint list can lead to an AttributeError when reading out duals. This is handled like in the Gurobi interface.

HiGHS:

• Added warning regarding #171

Tests:

• The QP test for unbounded problems was flawed. By reversing the sign of the objective function with quadratic terms, the problem became non-convex. This caused MOSEK to complain (matrix was not positive semidefinite) and was also the cause why "SCIP" needed special handling. I exchanged that with a new test case that is unbounded yet convex.

Environment:

• Was there a reason to not add xpress to the solvers in setup.py?

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Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (9985f1f) 86.88% compared to head (9dfb69d) 89.52%. Report is 3 commits behind head on master.

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Files	Patch %	Lines
linopy/solvers.py	60.00%	2 Missing and 2 partials :warning:

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[FabianHofmann]

This is awesome, thanks @fneum. The xpress installation does not seem to be available on mac for python 3.11. Perhaps we can find a way to ignore it for mac.