allow for str paths

[FBumann]

File "/Users/felix/Documents/Dokumente-eigene/Code/dresden/langfristmodell.py", line 187, in
flow_system.to_netcdf('flow_system.nc4')
File "/Users/felix/Documents/Dokumente-eigene/Code/dresden/.venv/lib/python3.11/site-packages/flixopt/flow_system.py", line 226, in to_netcdf
fx_io.save_dataset_to_netcdf(ds, path, compression=compression)
File "/Users/felix/Documents/Dokumente-eigene/Code/dresden/.venv/lib/python3.11/site-packages/flixopt/io.py", line 219, in save_dataset_to_netcdf
if path.suffix not in ['.nc', '.nc4']:
^^^^^^^^^^^
AttributeError: 'str' object has no attribute 'suffix'