- 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.

In Julia 1.11, the `Task` struct doesn't have a `logstate` field
anymore, causing unit test failures.

We use the `current_logger()` function instead, which is also
available in previous Julia versions (tested in Julia 1.6 to 1.11).

closes #109

closes #69

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

closes #40