Fix Irradiance/Irradiation

[johanneshiry]

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