Accept threshold values for calculating neighbours

[pfebrer]

As you may have noticed, I'm playing with categories :)

Right now, the AtomNeighbours category only works if you have a well defined basis for each atom, otherwise it throws an error because the maxR is smaller than 0.01.

What would be the best approach if I want to calculate neighbours for a geometry that I just created in sisl without specifying any orbitals? I think it is worth it to accept thresholds for each kind of atom in AtomNeighbours as you proposed in #218 (comment) and/or use defaults extracted from PeriodicTable.

And by the way I also agree that calculating neighbours based on orbital overlapping would be better :)