More abstractions for more generic and clearer code

[briochemc]

Long term goal! It would be great if I could literally call something like

∇ = Gradient(modelgrid; order, ...)
𝒖 = Advection(umo, vmo, modelgrid, ...)
𝐊 = IsopycnalDiffusionTensor(κH, κVML, κVdeep, mlotst, ρ, modelgrid, ...)
T = 𝒖 ⋅ ∇ - ∇ ⋅ 𝐊 * ∇

but I'm not sure how to not waste a lot of time into implementing such an interface, although there is certainly quite a bit of inspiration that can be taken from other packages (Oceananigans, MethodsOfLines, etc.)