Optimized PV inversion for two layer case in MultilayerQG

[glwagner]

Currently we implement "optimized" functions for extracting PV from the QG streamfunction, and vice versa, in the single layer case:

GeophysicalFlows.jl/src/multilayerqg.jl

Lines 485 to 495 in f4c68a5

	function streamfunctionfrompv!(ψh, qh, params::SingleLayerParams, grid)
	@. ψh = -grid.invKrsq * qh
	return nothing
	end
	function pvfromstreamfunction!(qh, ψh, params::SingleLayerParams, grid)
	@. qh = -grid.Krsq * ψh
	return nothing
	end

The inversion is also trivial in the two layer case, so we should support that.

To implement this I propose generalize SingleLayerParams to NLayerParams{N, ...}, where N is the number of layers. Then we have aliases

const SingleLayerParams = NLayerParams{1}
const TwoLayerParams = NLayerParams{2}

and we can implement optimized inversion routines for these two cases.

cc @szy21

[navidcy]

This is a duplicate or related to #112

[navidcy]

We should resolve this! It’ll make the MultiLayerQG so much faster!

[glwagner]

It's related to #112 but not a duplicate. #112 is about accelerating the arbitrary-layer case. This issue is about accelerating the two layer case.

[glwagner]

I think even if #112 is somehow resolved (eg by using KernelAbstractions --- a good solution for that issue still isn't clear to me), we probably still want to implement a special optimized method for two layer case.