Update simulations to a continuous phylogeographic process

[glstott]

A continuous phylogeographic process is needed in order to demonstrate the utility of the tool in comparison to existing strategies.

Instead of using diversitree to simulate our true tree, it is better to simulate both the trees and the sequences using SLPHY, which would keep things within a similar framework and allow for additional helpful characteristics to be added into our simulations. There is insufficient time to do this refactor before June, but opening an issue to upgrade before preprint is submitted. https://linguaphylo.github.io/setup/