Create modules for managing Crop and PracticalplantsCrop types

[petteripitkanen]

For cleaning up the code and working towards practicalplants integration, create module shared/crop.js for operating on the Crop type, and module shared/practicalplants.js for operating on PracticalplantsCrop type. PracticalplantsCrop module contains functions for converting to the powerplant Crop type, and it should be separated from the static practicalplants data and the file system.