Added MultiLayer QG Model.

[jejjohnson]

Added a 3-Layer QG demo (but it can be used with as many layers as needed).

I also ported a lot of useful functions that are more general within the library, e.g.

• (determinant) Jacobian for advection
• arbitrary wind forcing + Curl
• MultiLayer Domain class to help with initializing the matrices.

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Here is a demo. I copied the implementation from Louis Thiry (which borrows a lot of inspiration from Q-GCM)
Some stats:

• 3 layers
• 3840.0e3m x 4800.0e3m Square Basin
• 97x121 resolution
• 350., 750., 2900 - heights
• 0.025, 0.0125 - reduced gravities
• constant wind forcing
• Heun TimeStepper
• 2 year simulation
• 1,200 seconds - timestep
• saved every ~27 days
• 3 mins 30 secs runtime on my laptop (CPU)