- 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

This reverts commit b077630d.

`sow!(::ALMaSS.CropState, ...)` calls `setphase!`, which needs to know
which growth curve to use.  Some crop types (e.g. "permanent
set-aside") do not have a lownutrientgrowthcurve, which then causes an
error when `setphase!` tries to access the `missing` growth curve.

The highnutrientgrowthcurve would only be chosen when the cropstate
had `fertilising` in its events.  As the events for a cropstate are
now `empty!`ied in `sow!`, it's not possible to set the `fertilising`
event before calling `sow!`.

This previously didn't cause a bug, as the logic for which growth
curve to choose was the wrong way around, choosing the
highnutrientgrowth curve when no `fertilising` event was set for the
cropstate.

- When there are no fields `averagefieldsize` would return a unitless
  number.  This is now fixed.

- An unnecessary allocation was removed

- Fix unit tests, use approximate floating point comparison

- `stepagent!` for FarmPlot that delegates to crop state stepagent!
  function

- `harvest!`, `sow!` functions work on a `FarmPlot`, and delegate to
  the respective methods acting on a `CropState`

New `make_cropstate` helper function for switchable crop models.

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

Closes #9. Still needs testing & validation.

closes #85

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.

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 :-)

This commit includes a `sed` pass over all files to change all
occurrences of the former spelling "Persephone" to "Persefone":
`sed -ie "s/Persephone/Persefone/" *`