(Not yet tested.)

closes #40

closes #85

As well as the concomitant macros, and @killother

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

I now use parametric methods to store initialised parameter structs,
rather than creating a special registry for these with a closure.

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 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...

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.