closes #109

Searching for neighbours turned out to consume ~80% of processing time...

Closes #9. Still needs testing & validation.

closes #90

closes #85

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

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.

Major commit - not yet quite functional

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/" *`

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

Still needs to be tested.

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