fix: prevent cells from appearing multiple times in their neighbors list #55
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Summary
This PR fixes a bug with neighbor lists that appears when there are fewer than 3 cells in any given direction.
Each cell keeps a list of its neighbors, which includes the cell "on the right" and the one "on the left".
If there are only two cells in this direction, then the one "on the left" and the one "on the right" are the same (because of periodic boundary conditions).
The previous implementation would add this cell twice to the list of neighbors, resulting in pairs of interactions being counted more than once. This is removed by ensuring that all neighbor cell IDs are unique.
Notes
Trailing white spaces in the file are removed (initially because my IDE does that automatically), which I think is overall a good practice, but I can revert the change if you want.