Prune Population.js in preparation for Speciation

* Remove non-essential methods and functionality
* Standardize JSDoc tag sections
* Add population.species (property)
* Add population.speciate skeleton (method)
* Replace population.mutate with hard-coded Neat mutation spec (temporary)
* Partially replace population.evolve, include new .mutate functionality
* Add .evolve todos involving compatabilityDistance, speciation, and createOffspring
* Rework surviving methods to be (mostly) pure functions