Update cost currency to load dynamically from the user's Pod Point account

[mattrayner]

Is your feature request related to a problem? Please describe.
I would like to automatically define the currency of our cost sensors based on the return from Pod Point rather than defaulting to GBP and allowing the user to set it.

Its un-necessary logic in the integration and should be cleaned up

Describe the solution you'd like
Looking at the user data returned from podpointclient, we should use the attributes returned from the Pod Point APIs to drive the currency data in PodPointTotalCostSensor and it's children

Describe alternatives you've considered
N/A

Additional context
This has become possible thanks to the changes in podpointclient