Skin style neighborlist

[CheukHinHoJerry]

Hi developers,

I have implemented a simple version of lammps-like skin layer for neighbourlist in here:

https://github.com/CheukHinHoJerry/torch-dftd

(see Readme for a short summary). The motivation of having this is to prevent the dispersion correction being the bottleneck when using with ML-potentials. I think it would be in general helpful to have to this feature. I have a non-exhaustive list of tests but would be happy to add more and do a clean up + make sure to have consistent coding style.

I would create a PR if that is of interest?