Convert SUN_IRRADIANCE Unit from W*h to kJ

[fl0x53]

Convert reported 'global_radiation_last_hour' from W*h/m^2 to kJ/m^2 by multiplying with 3.6

[FL550]

Hi, as DWD is delivering this value in this format, I'd rather like to leave it this way. Or do you have any specific reasons for your PR?

[fl0x53]

Yes, I do have 😄:
There is a unit mismatch when using get_forecast_data() vs. get_reported_weather() for attribute SUN_IRRADIANCE
The first uses Rad1h in kJ/m^2 (as of MOSMIX description), the latter global_radiation_last_hour in W/m^2 per one hour ("letzte Stunde").

For example, this leads in HA to values that are 3.6 times smaller than expected.

[FL550]

Ah, I see! There was a problem regarding this some time ago.

Thanks for your PR!