- `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

(Not yet tested.)

closes #85

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 used to be done by Agents.jl, is now done natively

I now use parametric methods to store initialised parameter structs,
rather than creating a special registry for these with a closure.

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.

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

This closes #24.

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 :-)

It is now contained within a single function that can be called at
need, rather than being calculated every day for every pixel.

Closes #52