- each crop model must implement types `CropType <: AbstractCropType`
  and `CropState <: AbstractCropState`

- `AgricultureModel` and `FarmPlot` are not parametrised structs
  anymore to allow for different crop models active in one simulation

closes #40

closes #85

As well as the concomitant macros, and @killother

This is a huge commit that changes much of the underlying model
architecture (which is why it all had to change at once). Agents.jl
is now no longer a dependency and the species macros have been largely
rewritten. Not everything has been adapted yet, but the model is in a
"minimally runnable" state.

Also moved the DSL macros into a separate file. Still need to write
tests for all of this.

This closes #47

Compile time still increased to 50s (from 15s), but at the moment I
can just about live with that.

This closes #24.

closes #64