In Julia 1.11, the `Task` struct doesn't have a `logstate` field
anymore, causing unit test failures.

We use the `current_logger()` function instead, which is also
available in previous Julia versions (tested in Julia 1.6 to 1.11).

closes #40

closes #85

Also updated CONTRIBUTORS.md, added a "none" option for
`core.logoutput`, and added type annotations to the Skylark struct

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.

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

This closes #24.

closes #64

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/" *`

This closes issue #23.

Multiple runs and parameter scanning works, but parallelising was (as
always) a bit trickier than expected... Will try to get this working
soon, but for now I need to focus on more important things.

Previously, a global logger was used, which would have given problems
on a parallel run. Also split up the `setupdatadir()` function to
improve code structure.

This is a major commit with quite deep-reaching changes in the code
base. It enables the user to specify multiple values for a model
parameter, which will then be combined into a full-factorial
simulation experiment. Everything seems to work as intended, although
some refactoring would be good, and we still need to add parallelisation
for issue #42.

And yes, I coded all of this between 23:30 and 03:00 on a Saturday
night  

This is a major change to the codebase. I'm not entirely convinced yet
that it actually makes sense, but overall I think the benefits
outweigh the problems. Closes issue #37.

This is not yet in a working state.

Still needs to be tested.