Fix bug in CropBuilder where N, P and K quantities must be always specified

[mcmancini]

The current version of the CropBuilder to run WOFOST in bulk (i.e., the class that allows to build the parameter structure to override devault parameters in WOFOST based on user input) at the moment is not flexible enough in the definition of the fertilisation events. Fertilisation events must be defined as timings of NPK fertilisation events, and associated quantities of nutrients for each timing event. The way I intend for this to work is to preload default agromanagement in terms of number of fertilisation events, timing and quantities, and then to override only the values of interest, if any. At the moment, however, the script does not take default values and breaks if some of the timings or quantities are not being defined by the user through the input csv file.
Changes will need to be made for the _generate_args() method in the CropBuilder class, lines 435 and following.