Fix Irradiance/Irradiation

Currently the old, invalid version of irradiance and irradiation is used within the PSDM. 
This affects the `StandardUnits` as well as several other places. 

The following changes are needed:

- adapt `StandardUnits` to use irradiance and irradiation correctly
- adapt `StandardUnits` to use `WATT_PER_SQUAREMETER` as standard irradiance and `WATTHOUR_PER_SQUAREMETER` as standard irradiation