Multigrid / coarse graining

[briochemc]

It would be great to be able to coarse-grain any given matrix to allow for quick testing.

Not sure how to do this easily. In particular, making this generic might not be possible, in particular, if the fine grid has a non-easily divisible size in one dimension (e.g., OCIM2 has size 91 in latitude, which is divisible by 7 🤷), or if the coarse grid would bundle separate boxes together (e.g., boxes originally separated by land diagonally).