[ENH] set neighbour as a parameter

[andreifoldes]

For AFNI users it could be useful to be able to specify how according to which rule to group voxels!

https://afni.nimh.nih.gov/pub/dist/doc/program_help/3dClusterize.html

-NN {1|2|3} :Necessary option to specify how many neighbors a voxel
has; one MUST put one of 1, 2 or 3 after it:
1 -> 6 facewise neighbors
2 -> 18 face+edgewise neighbors
3 -> 26 face+edge+cornerwise neighbors
If using 3dClustSim (or any other method), make sure
that this NN value matches what was used there. (In
many AFNI programs, NN=1 is a default choice, but BE
SURE YOURSELF!)