fix: also create parents during export directory creation (#197)

If the export directory doesn't exist, it will be created. However, this
fails if the parent also doesn't exist (e.g
`data/steel/output/transience/pickle` will fail if there is no
transcience folder yet).

This is fixed here. In a follow-up, one could probably migrate more of
the older `os.path` functions to use `Path`.

Author: tobiasdiez

remind_mfa/common/common_export.py

@@ -364,7 +364,7 @@ def export_path(self, dataset: str, filename: str | None = None) -> str:
 
         base_dir = os.path.join(*path_tuple)
         if not os.path.isdir(base_dir):
-            os.mkdir(base_dir)
+            Path(base_dir).mkdir(parents=True, exist_ok=True)
 
         if filename is not None:
             path_tuple += (filename,)