Remove "convert" method on datasets

[BSchilperoort]

The convert method is not used at all, or useful in any way.

Datasets are loaded by the recipe runner, and the datasets are then converted using zampy.datasets.converter.

[geek-yang]

Some historical information about zampy.datasets.converter. I think at the beginning we want users to use zampy via python API (e.g. call the functions of datasets inside a notebook). And we want users to call convert function to convert the ingested data and save it on disk. But at that moment, we are not sure whether it makes more sense for the users to save the converted data to disk, or simply keep everything in memory for other tasks (e.g. run model, or do some computation). That's why the function is quite empty and we simply grab zampy.datasets.converter in the demo notebook, as a temporary solution. This way it also has no added value to the recipe runner.

But if we want to give the option for the users to convert the ingested data (to the target grid) and export the converted files somewhere (automatically), we might still need it.