Add `netCDF4` dependency

After having aiida-flexpart plugin (v0.1.3) I hit an error:

```python
  File "/home/jovyan/.local/lib/python3.9/site-packages/aiida_flexpart/workflows/inspect.py", line 6, in <module>
    from netCDF4 import Dataset
ModuleNotFoundError: No module named 'netCDF4'
```
That should be fixed by adding `netCDF4` as a dependency of the plugin.