Reimplement LinkedCellList with Cajita

[streeve]

Continuing from #197, using Cajita the LinkedCellList can be updated and the underlying CartesianGrid replaced.

The LinkedCellStencil within the VerletList for looping over neighboring grid cells can then also be replaced.

• [ ] Separate core neighborlist algorithm from grid accelerated
• [ ] Create Cajita-dependent versions of LinkedCellList and VerletList
• [ ] Deprecate core library versions and CartesianGrid