[ModelicaSystem/ModelicaSystemDoE] improve session handling

* add ModelicaSystem.session() - returns _getconn
* add ModelicaSystemDoE.session() - returns _mod.session()

reasoning:
* do not access private variables of a class
* limit chain access to (sub)data

Author: syntron

OMPython/ModelicaSystem.py

@@ -439,6 +439,12 @@ def __init__(
         if build:
             self.buildModel(variableFilter)
 
+    def session(self) -> OMCSessionZMQ:
+        """
+        Return the OMC session used for this class.
+        """
+        return self._getconn
+
     def setCommandLineOptions(self, commandLineOptions: str):
         """
         Set the provided command line option via OMC setCommandLineOptions().
@@ -1858,9 +1864,9 @@ def __init__(
         self._timeout = timeout
 
         if resultpath is not None:
-            self._resultpath = self._mod._getconn.omcpath(resultpath)
+            self._resultpath = self.session().omcpath(resultpath)
         else:
-            self._resultpath = self._mod._getconn.omcpath_tempdir()
+            self._resultpath = self.session().omcpath_tempdir()
 
         if not self._resultpath.is_dir():
             raise ModelicaSystemError(f"Resultpath {self._resultpath.as_posix()} does not exists!")
@@ -1873,6 +1879,12 @@ def __init__(
         self._sim_dict: Optional[dict[str, dict[str, Any]]] = None
         self._sim_task_query: queue.Queue = queue.Queue()
 
+    def session(self) -> OMCSessionZMQ:
+        """
+        Return the OMC session used for this class.
+        """
+        return self._mod.session()
+
     def prepare(self) -> int:
         """
         Prepare the DoE by evaluating the parameters. Each structural parameter requires a new instance of
@@ -2014,7 +2026,7 @@ def worker(worker_id, task_queue):
                     raise ModelicaSystemError("Missing simulation definition!")
 
                 resultfile = mscmd.arg_get(key='r')
-                resultpath = self._mod._getconn.omcpath(resultfile)
+                resultpath = self.session().omcpath(resultfile)
 
                 logger.info(f"[Worker {worker_id}] Performing task: {resultpath.name}")