Explicitly use simplex solver with HiGHs for better parallelism

Author: gthomsen

src/pownet/modeling/model.py

@@ -92,6 +92,7 @@ def _optimize_highs(
         self.model.setOptionValue("mip_rel_gap", mipgap)
         self.model.setOptionValue("time_limit", timelimit)
         self.model.setOptionValue("threads", num_threads)
+        self.model.setOptionValue("solver", simplex)
 
         self.model.run()
         # Delete the MPS file