Review dataloaders

[thomaszwagerman]

Review opportunities for generalising data loaders.

Considerations

• General approaches (e.g. gebco.py) renaming lat/long variable names to conform with meshiphi.
• Redundant loaders scotland_ncmpa.py, baltic_sea_ice.py, baltic_current.py, north_sea_current.py?
• Loaders where their purpose is not clear/self-documented (thickness.py)