Fix #811

[jebob]

This should fix #811? Sadly, I don't have working tests (see #840) or access to the solvers.

[pchtsp]

Thanks. Can you add a unit test with the error you're trying to fix? that way we're sure if it actually solves it or not.

[jebob]

I've added a test, but I cannot verify that it triggers an error (as I don't have Gurobi locally). I can't trigger the Github pipelines myself, so I can't test the test with and without the fix.

[pchtsp]

You can install gurobi locally by installing

pulp[public_py]

This will install the public python APIs, including gurobipy.

[pchtsp]

And you need to format with black

[jebob]

You can install gurobi locally by installing

pulp[public_py]

This will install the public python APIs, including gurobipy.

Aha, this installs the GUROBI solver, whereas I've patched the GUROBI_CMD solver, which is why I am struggling to reproduce the bug locally. Nonetheless, I think the test is correct?