Add CUDA support to growth.jl settler_cover

[arlowhite]

Takuya has seen ~250x speedup experimenting with GPU support here.

Code should use CuArray automatically if within a GPU environment.

[ConnectedSystems]

Potential x270 improvement (2.100805 / 0.00777 = 270)

The 75 iterations is a proxy for the number of time steps being simulated.

In practice the difference may not so large, particularly as we're not looking at all reefs (we subset to only those that are source/sink locations), but worth a crack.