text: Title: [ENH] Remove `gluonts` dependency in `gluonts_ListDataset_panel` mtype
  Body: The `gluonts_ListDataset_panel` data type specification does not require `gluonts` objects, as it is composed entirely of `python` base types and `pandas` types.
  
  Therefore we should be able to remove the `gluonts` dependencies. To do this:
  
  * look at the `PanelGluontsList` class and replace `gluonts` imports by equivalent checks that do not require `gluonts`
  * remove `gluonts` from the `python_dependencies` tag