Adding xpress postsolve if necessary (#421)

Co-authored-by: Fabian Hofmann <[email protected]>

Author: come-bissuel

linopy/solvers.py

@@ -1581,6 +1581,10 @@ def solve_problem_from_file(
 
         m.solve()
 
+        # if the solver is stopped (timelimit for example), postsolve the problem
+        if m.getAttrib("solvestatus") == xpress.solvestatus_stopped:
+            m.postsolve()
+
         if basis_fn is not None:
             try:
                 m.writebasis(path_to_string(basis_fn))