closes #69

closes #40

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.

Unfortunately, including Makie as a dependency increases the compile
time to ~5 minutes. This is unacceptable and I need to find a fix for
it, if possible.

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/" *`

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 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.

Translated the Wolpertinger to the new syntax, Wyvern to follow later.

Not yet functional!