Add DistributedTree Nearest query with callback

[masterleinad]

The strategy here is to do a regular query that gets us the MPI ranks, and primitive indices for each query, send them back to the MPI rank owning the primitives, execute the callback, and send them back to the MPI rank owning the queries.

[masterleinad]

This should work now replaying the found matches on the remote MPI rank and communicating the results back. Currently, I don't treat the default callback special to validate that this behaves as expected.

[masterleinad]

This should be ready for inital review. As said before, the strategy here is to do a regular query that get's us the MPI ranks, and primitive indices for each query, send them back to the MPI rank owning the primitives, execute the callback, and send them back to the MPI rank owning the queries.

[masterleinad]

I will still adopt some more tests to use the callback.

[masterleinad]

@aprokop ping

[masterleinad]

We need to support pure callbacks They would use mostly the same code, except the API would not have offset and output views, and they won't need the extra round of backward communication

That's the same as https://github.com/arborx/ArborX/pull/733, right?

[aprokop]

That's the same as #733, right?

No, that was for spatial queries. I need to think a bit about that one.