Netcdf conversion failing

My refactor of the netcdf seems to have broken.
```
ValueError(TaskFailure(worker=h0.w4, task='convert_to_netcdf:7951227359fbd9e5a98f7a2ce2fb7f378eec3f895541fc5bf8f23885cc6eaefa', detail="ValueError('I/O operation on closed file.')"))

```

It looks as if the earthkit exporter closes the file, so we should convert to xarray and then dump to bytes.