Add Capabilities #6

Datapoint provides capability information. This states what data is available for a specific site or sites without returning the data itself.

This would be useful for error checking and setting bounds rather than assuming a 5 day forecast will be returned.

http://www.metoffice.gov.uk/datapoint/product/uk-3hourly-site-specific-forecast/detailed-documentation#5,000 UK locations three hourly forecasts capabilities feed