CadsObs adaptor to use the retrieve_list_of_results method

[EddyCMWF]

For consistency with other adaptors, the CadsObs adaptor should use the retrieve_list_of_results method.

This returns a list of paths, and is called by the default self.retrieve method. The self.make_download_object is then responsible turning the list of paths into a downloadable object for the retreive-api.

If the default self.make_download_object is incompatible with the adaptor, then please define your own retrieve method which handles the creation of a download object, which must be an open binary file.

This will give more flexibility with anticipated functionality, and allows use with the MultiAdaptor.

(Markel, this is lower priority than the task that Paul has given you, we can discuss in the next sprint meeting)

[garciampred]

It should be easy to do, we only need to be careful with the error handling.