Initial import
Removed perennial_system() and annual_system() as part of movement to using the grain_system class.
Moved state inside of grain class
Moved events queue inside of grain class
Placed discrete_event_handler() closer to the event-related definition
Relocated write_system() function above grain_system class so that it is available from inside the class.
Could use a forward declaration instead
Encapsulated loop for executing grain simulation inside grain_system class.
Improved comment formatting
Moved time functions into a separate function so that the grain system can be recycled
Set up for doing parameter-space searches