Add API for setting the outside temperature

[yorickpeterse]

This adds support to V2 of the API to set the outside temperature of an Itho WPU. This allows controlling of this value without the need for a physical sensor (i.e. by using the Buienradar API).

I didn't include support for the temporary temperature value and timestamp. The use case is to have some sort of script run periodically and update the temperature according to an external source, in which case the temporary temperature value and timestamp isn't useful. If it's desired to add this anyway, I'll happily add it of course :smiley:

[arjenhiemstra]

Thanks! No need to add support for the temporary temperature value and timestamp atm. If there is a need in the future someone can add it easily.