Implemented new optional combined water and nitrogen stress on plant photosynthesis.
The new option is made available by using the
PlantStess switch and when using Radiation use efficiency as Growth model
A new parameter is named RelW Water that is defined in the parameter table: Photosynthesis Response
The combined response is calculated as
Response=(a*sqrt(ResponseWater)+(1-a)*sqrt(ResponseN))**2 PEJ