CamsSolarRadiation adaptor to use retrieve_list_of_results method

[EddyCMWF]

For consistency with other adaptors, the CamsSolarRadiation 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.