Add method to return all GSPs and descriptions

[PhilPJL]

To allow a user to select their GSP manually (if postcode and mpan methods don't work), provide access to the collection of GSP identifiers along with a description, e.g.

'A', 'East England',
'B', 'East Midlands',
etc

https://en.wikipedia.org/wiki/Distribution_network_operator#History