[ModelicaSystem] improve result file handling in simulate()

Author: syntron

OMPython/ModelicaSystem.py

@@ -1110,10 +1110,12 @@ def simulate(
         if resultfile is None:
             # default result file generated by OM
             self._result_file = self.getWorkDirectory() / f"{self._model_name}_res.mat"
-        elif os.path.exists(resultfile):
-            self._result_file = self._getconn.omcpath(resultfile)
+        elif isinstance(resultfile, OMCPath):
+            self._result_file = resultfile
         else:
-            self._result_file = self.getWorkDirectory() / resultfile
+            self._result_file = self._getconn.omcpath(resultfile)
+            if not self._result_file.is_absolute():
+                self._result_file = self.getWorkDirectory() / resultfile
 
         if not isinstance(self._result_file, OMCPath):
             raise ModelicaSystemError(f"Invalid result file path: {self._result_file} - must be an OMCPath object!")