Different regridding methods

[asinghvi17]

Currently we just have first-order area-conservative regridding. But many others are possible using this framework if slightly generalized:

• 2nd order conservative regridding - like 1st order but gradient aware, just needs a more clever weight formula
• Nearest neighbour interpolation - simple enough, just find the closest centroid or the polygon with the most overlap - can have two modes
• Natural neighbour interpolation - similar to 2nd order conservative regridding, I think this can be slotted in relatively easily (I think). We could use a lot of different formulas that are implemented in e.g. NaturalNeighbours.jl. But I'm not entirely sure if the math works out yet - need to think about that.
• Polynomial patch recovery interpolation (like ESMF) - more complex at the regridding step since it needs to effectively integrate all neighbour patches