Non periodic neighbour search keeps track of particles that leave the box

[LonelyCat124]

At the moment when a particle moves out of the box, the particle remains in the cell it previously belonged to, meaning it still can interact with the rest of the system if in range.

I don't think this is how it should work, however I don't want to worry about how this boundary should actually be implemented yet - I think a particle leaving the box is currently regarded as undefined behaviour.

[LonelyCat124]

I think this is a physics problem, and mostly the question is how to give this data to the users so they can make use of it (for e.g. sink boundaries) etc.