- 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

- no majority counting of Pixels implemented yet, soil type is
  currently still determined by the first pixel of a FarmPlot

- `Pixel` now has a field `soiltype`

- read soil type information from tiff file

Rename this function to `showlandscape()`.

`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.

Now include cloud cover, humidity, and ETo, dropped vapourpressure

The data/crops/almass/crop_data_general.csv file now stores a column
`phase_before_harvest` to indicate in which growing phase the plant
can be harvested.  This is `marchfirst` for most crops, and `sow` for
maize.

In theory, with the current setup there could be a problem for
peas/beans if they are planted after March, 1 (the planting dates are
15 February to 15 March).  Planted before March 1st, the last growth
phase before harvest would be `marchfirst`.  But if planted after
March 1st, the last growth phase before harvest would be `sow`.  The
GDD curves for this crop only exist for `sow`, so probably planting
before March 1st should not be allowed for this crop.

Currently the sowingdensity column is still treated as type `String`,
as there is an entry of "35/80".  Once this entry is clarified, the
type should probably be `Float64`.