Improve way in which input parameters are overridden

[mcmancini]

There are currently the following ways to generate and/or override parameters based on their type:

• agromanagement through the Crop class passing either default agromanagement loaded from file (see defaults.py) or through an instance of the CropBuilder parametetr builder class.
• WOFOST parameters and initial soil conditions through the run() method in the WofostSimulator class. Any of the parameters in the list contained in defaults.py defaults.wofost_parameters can be modified using the built-in _override_defaults() private method.

The aim for this issue is to replicate a way to alter parameters for crop selection and management in a similar way as implemented in WofostSimulator. At the moment, for example, default parameters are not provided automatically; there is no list of parameters that can be modified and there is not method similar to _override_defaults(). While the code works, it is clunky.