Fix pushpull backward when D > 3 #4
Description
The fallback implementation when D is not 1, 2 or 3 is in pushpull/nd.h.
I looked at the file by chance, and realized that while the forward passes look OK, the backward passes are just copy-pasted from the 3D case, which of course does not make sense. It should be fixed (and added to the test suite).