Return a tile that has already been generated instead of running it again #39

Background

Generating a tile is expensive and the visualizer will ask for the same tile more than once as the user moves around.

Description

Before generating, ask the tile cache from #31 whether it already holds a finished grid for this tile seed and return that if it does. Otherwise generate it, put it in the tile cache, then return it.

A tile is identified by its seed, which already has the coordinate worked into it. Two different global seeds are two different worlds and must never share a tile.

Testing:

• Assert asking for the same tile twice only runs the sampler once
• Assert the second call returns the same grid as the first
• Assert the same coordinate under a different global seed runs the sampler again