[ModelicaSystem] use OMCPath for nearly all file system interactions

remove pathlib - use OMCPath and (for type hints) os.PathLike

Author: syntron

OMPython/ModelicaSystem.py

@@ -38,7 +38,6 @@
 import numbers
 import numpy as np
 import os
-import pathlib
 import textwrap
 from typing import Optional, Any
 import warnings
@@ -118,7 +117,7 @@ def __init__(
             timeout: Optional[float] = None,
     ) -> None:
         self._session = session
-        self._runpath = pathlib.Path(runpath).resolve().absolute()
+        self._runpath = runpath
         self._model_name = modelname
         self._timeout = timeout
         self._args: dict[str, str | None] = {}