WIP: Add LinkedCellList parallel_for

[streeve]

Add an interface for looping through a linked cell list as if it were a neighbor list. Part of #460. Still WIP:

• [ ] Probably makes more sense to rename neighbor_parallel_for
• [ ] Would prefer to store a stencil inside the LCL instead of passing them separately
• [ ] Started adding the neighbor list traits, but not using them yet
• [ ] Needs more robust testing