Change Irradiation's Unit Dimension to Power/Area

[mia-krause]

As discussed, the dimension of Irradiation should be changed from Energy/Area to Power/Area. The standard unit is now kW/m² instead of kWh/².
This change holds two benefits:

1. Irradiation now represents the common dimension for solar irradiance
2. There is no ambiguity about the time frame and context of values

PowerDensity, which also represents Power/Area, will be removed.