v0.2.0.9000

Main changes :

• Simplified the required interface for model wrappers (i.e. shape of input and output arguments, see the Guidelines for implementing a crop model R wrapper for CroptimizR)
• Added a new argument var_names to estim_param function for requiring the model wrappers to simulate non-observed variables (may be useful if transform_sim / transform_obs arguments are used)