Add AquaCrop.jl crop model, and support multiple crop models in a simulation

Closes #122

See merge request persefone/persefone-model!6

- 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()`.

Remove duplicated data files from test folder

Closes #118

See merge request persefone/persefone-model!5

Closes #118

Change weather data representation

See merge request persefone/persefone-model!4

`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.