(Not yet tested.)

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

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.

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.

Also moved the DSL macros into a separate file. Still need to write
tests for all of this.

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 #50

This allows us to get rid of the annoying `animal.traits["traitname"]`
syntax and replace it with the much simpler `animal.traitname`.

Using the GLOBAL_RNG introduces global state, which must be avoided
to preserve reproducibility. Therefore, all Persephone code must use
`model.rng` whenever calling `rand()`/`shuffle!()`/etc.

This seems to work as intended now! :D

This is a major commit with plenty of bug fixes. Turns out, macrology
is *hard*! Not everything works yet, there are still several important
FIXMEs scattered throughout the code. But at least the macro
expansion/module scoping that has been bugging me for most of the past
week has been (mostly) fixed now, or at least avoided. We're getting there...

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.

Everything now seems to work as intended :-) (Although a lot of the
nature tests are still missing.)

Still errors - looks like `@habitat` still needs fixing.

Still needs to be tested.

Not yet functional - the necessary functions still need to be implemented.