Neighbor execution policy

[streeve]

#414 points out that serial neighbor indexing does not work when using a work tag with the neighbor_parallel_for because that tag is injected and expected in the internal lambda (in addition to the user kernel).

• [x] Create a new execution policy inside neighbor_parallel_for with void tags directly
• [ ] New linkedcell_parallel starts to build the necessary structs #712
• [ ] Create a NeighborPolicy analogous to the SimdPolicy and simplify the function interfaces (this will handle the work tags and can also hold tags like neighbor order and parallelism level)
• [ ] Deprecate existing functions without NeighborPolicy

[streeve]

Note that #480 is the right place to start for the rest of this work