Writing a selafin mesh dataset to a file path with special characters fails under windows

[nstrahl]

Using file paths for non-ANSI characters (äöü) is not handled properly by the selafin driver in MDAL::SelafinFile::addDatasetGroup.
This can be quickly verified in QGIS, where the native:tinmeshcreation algorithm will run without error. However, the resulting mesh will only contain the mesh structure without the "elevation" dataset.