Add events handling

[rafaqz]

Often we have know events that occur over the period of a simulation. These may be historical artifacts that cannot be directly modeled, or things that arrive from outside the system we are modelling.

These are probably best organised as a DimVector with a time dimension that can be synced to the simulation, holding NamedTuples where the key is the grid name and the value is a Vector of point/value pairs. Or something like that. Then we can set any point on any grid to any value, for any timestep. We could also have events that had a funciton that was applied to each point in the vector.

@mkborregaard I had the idea to use these events to trigger historical changes in land use cover change models. So maybe when a settlement was founded is an event, especially when it comes from somewhere outside the simulation.

Should probably also look at how DiffEq handles events...

[mkborregaard]

sounds great