Introduce switchable crop models

src/Persefone.jl

src/core/input.jl

src/core/simulation.jl

src/crop/almass.jl

src/crop/cropmodels.jl

src/crop/farmplot.jl

src/crop/simplecrop.jl

src/farm/farm.jl

src/parameters.toml

test/crop_tests.jl

test/nature_tests.jl

test/paramscan.toml

test/runtests.jl

test/test_parameters.toml

test/test_parameters_almass.toml

test/test_parameters_simplecrop.toml