Allow for selection of Agile Tariff

[LucidityCrash]

As of July 2022 there are now 3 Agile Tariffs :

• AGILE-18-02-21 - The unit rate is capped at 35p/kWh (including VAT)
• AGILE-22-07-22 - The unit rate is capped at 55p/kWh (including VAT)
• AGILE-22-08-31 - The unit rate is capped at 78p/kWh (including VAT)

this allows selection of which rates to fetch default if not set is AGILE-18-02-21

Addresses #96