Add elevation impacts for plate tectonics

[anyweez]

Since the latest refactor, tectonic plates no longer affect cell elevation. Simplex noise is the primary contributor to elevation, which lacks the "continental" look of the previous plate-based approach.

For reference, here are the Earth's tectonic plates:
https://en.wikipedia.org/wiki/Plate_tectonics#/media/File:Plates_tect2_en.svg

There are two specific properties of plate tectonics that I want to try out:

1. Individual plates may be higher in elevation than others. All cells of certain plates should start with a base elevation.
2. Each plate boundary may have certain elevation-impacting features, depending on the boundary type (divergent, convergent, transform).