Compute IIS if problem is infeasible for solver COPT

COPT has a python example of how to calculate the IIS to help debug infeasible solutions. It would be great to add this functionality to linopy.

Compute IIS if problem is infeasible

if model.status == COPT.INFEASIBLE:

Compute IIS

model.computeIIS()

Check if IIS is available

if model.hasIIS:
# Print variables and constraints in IIS
cons = model.getConstrs()
vars = model.getVars()

print("\n======================== IIS result ========================")
for con in cons:
  if con.iislb or con.iisub:
    print('  {0}: {1}'.format(con.name, "lower" if con.iislb else "upper"))

print("")
for var in vars:
  if var.iislb or var.iisub:
    print('  {0}: {1}'.format(var.name, "lower" if var.iislb else "upper"))

# Write IIS to file
print("")
model.writeIIS('iis_ex1.iis')

Start by reviewing the COPT Python IIS example in the issue and then locate linopy's solver integration for COPT. Determine how infeasible solutions are represented and where solver diagnostics are exposed. Done means linopy can compute and report an IIS for infeasible COPT problems, including the relevant constraints and variables.