Allow stics_wrapper to handle param_values for different plants in intercrops

[sbuis]

This would require:

• adopting a convention for naming (plant) parameters depending in the plant number (_Crop1/2 is used in SticsRFiles for xls2xml transformation)
• checking that the parameter is a plant parameter and that the corresponding plant is simulated (if plant#==2)
• adapting SticsRFiles:::force_param_values/gen_paramsti (set plt to the plant number before modifying the plant parameter)