Automatically add padding to FullGridCellList bounding box?

[efaulhaber]

When a particle is in a boundary cell of the grid, it is not surrounded by cells, so we get an out-of-bounds exception when looking for neighbor cells. Currently, one has to manually add a padding of search_radius to the bounding box.
I think it would be better if the padding was automatically included (only when not using periodicity?) and maybe could be deactivated with padding = false.

[sloede]

Good idea! Maybe the padding could be automatically based on the maximum search radius (I don't know if this can be specified) in case of variable search radius setups?

[efaulhaber]

If we want to do variable search radius with p4est, we don't need a padding, as p4est gives us the actual neighbor cells.