Just jumping into this PR discussion cold ... :snowflake: 

Adding ```var_name``` to the ```CoordDefn``` and ```CubeMetadata``` has direct implications for the cube merge (that's stack and concatenate new style) e.g. two cubes, one from NetCDF and one from PP, superficially they should merge but they certainly won't due to ```var_name```. Should merge cope with this?

NetCDF time series data from disparate sources in separate files won't merge if the ```var_name``` for the underlying data is not in agreement. This is a subtlety that the user won't appreciate. Again, should merge cope with this?