Interface of data and era5 modules

[JaroCamphuijsen]

It would be good if the interface of the data and era5 modules resembles the other model interfaces more to enable straightforward coupling of models with data.

Suggestions (please add):

• [ ] They should generate their own flow links and nodes
• [ ] Merge data and era5 modules together. Providing an option to download era5 on the go from CDS, or provide own locally stored data files
• [ ] Rename data module to ExternalData
• [ ] Try to retrieve units from 'attributes" of the loaded netcdf.

[Ireneben73]

Not sure if this is part of the interface module, but it would be great if you could just save part of the output grid (certain location/extent) by defining a bounding box, for all time steps. Now I do it myself, but using a bulky code that slows down a lot the run.. I think this enhancement should go in another section of github maybe