- 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

- `Pixel` now has a field `soiltype`

- read soil type information from tiff file

`Weather` is now a type that stores the complete simulation data
(previously it just stored the weather data for a single day).
Weather data storage is now a struct-of-arrays, previously it was a
dict-of-structs.

Weather data can be accessed with functions such as `sunshine(weather,
date)`.

Missing weather input data for temperature (min/max/mean),
precipitation, and evapotranspiration is now an error.  In the future,
missing values could perhaps be imputed.

`AgricultureModel` and `Weather` are now defined with `@kwdef`,
allowing their constructors to be called with keyword arguments.

closes #109

This allows the removal of the abstract types AbstractFarmPlot,
AbstractCropType, AbstractCropState.

Also updated CONTRIBUTORS.md, added a "none" option for
`core.logoutput`, and added type annotations to the Skylark struct

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.

Still need to write tests to make sure it works properly, but at least
it runs without errors now and seems to do what it's supposed to.

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

closes #68

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

This closes #24.

closes #64

This is a major commit, probably rather too large, but anyway. There
are no tests yet, so no guarantee for correctness. But it's good
progress :-)