Add efficient parallelism

[eliotwrobson]

See title. The current parallel code doesn't perform well in benchmarks (for some unknown reason, probably something with the memory access being weird) and will be removed for the first alpha release. It should be possible to add back this functionality later on. Reference to a commit with this code on it:

https://github.com/eliotwrobson/ndleafy/blob/d21c628a591f186579a2776b5cbcd0bc6d661df3/src/ndleafy/models.pyx#L131