neigbors method return negative numbers

[vlccek]

Thank you for providing this incredible library.

I'm currently using the development branch of voro to take advantage of its parallel processing features. However, I've come across a challenge with the neighbour identification feature, which occasionally returns negative values. Are these negative numbers indicative of a specific issue, or an overflow?

[chr1shr]

Negative values in the neighbor list correspond to Voronoi faces that:

• touch the domain boundary, in a non-periodic domain,
• are due to images of the current particle, in a periodic domain.

This issue is discussed in some of the documentation, e.g. the description of neighbor lists under the %n option on the custom output reference.