Add lip factor to neighbor list cells

[AhmedSalih3d]

Motivation

• Reduce the frequency of neighbor-list rebuilds by making cell boxes artificially larger (Verlet-style overlap) so neighbor lists remain valid longer.
• Allow overlapping boxes and scale them from cell centers using a lip factor (default 1.2) so the verlet methodology is preserved.

Description

• Add DefaultNeighborLipFactor = 1.2 and NeighborListInverseCutOff(InverseCutOff, LipFactor) to src/SPHNeighborList.jl to compute an effective inverse-cutoff for enlarged cells.
• Change ExtractCells! to accept a LipFactor and use the effective inverse-cutoff when mapping particle positions into cells, and make UpdateNeighbors! accept a LipFactor keyword and forward it to ExtractCells!.
• Update src/SPHCellList.jl to import DefaultNeighborLipFactor and NeighborListInverseCutOff, apply the lip factor when mapping ghost points to cells, and scale the neighbor-list rebuild threshold and UpdateNeighbors! call by the lip factor.

Testing

• No automated tests were run for this change.
• Commands executed during the change were git add src/SPHNeighborList.jl src/SPHCellList.jl and git commit -m "Add lip factor to neighbor cells".

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