More Green functions

[cortner]

Would be good to generalise the current green's function implementation to

• 2D -> 1D
• 2D -> 2D
• 2D -> 3D
• 3D -> nD

[thudso]

3D -> nD should be possible relatively easily (in fact the code should function almost the same, it just needs some manipulation of types throughout), and if @jjbraun's 2D formula works correctly, we can also do the 2D -> nD versions in a similar way.