ArborX neighbor search

[kuberry]

Add ArborX as an option for neighbor search, enabling GPU accelerated neighbor search.

What has been done:

• Get radius search working (CPU/GPU)
• Get KNN search working (CPU/GPU)
• Add ArborX source files
• Refactor tests to work with floating point neighbor search

What remains:

• Move ArborX source files into tpls folder
• Make neighbor searches do parallel work in execution space corresponding to memory space of vectors provided (if any have a device memory space, use this)
• Switch search interface to pass by reference rather than pass by value, enabling resizing of neighbor lists in the search call. Replace the dry run / real search process with a single call that does both.
• Replace CPU sort routine in ArborX with updated version

This PR changes the API for neighbor search with respect to the location of is_dry_run becoming do_dry_run, getting a default value of true (never to be changed manually by a user).

[aprokop]

Overall, I think the code in Compadre_PointCloudSearch_ArborX.hpp looks fine. I think you do some unnecessary deep copies, though. For example, in both generateCRNeighborListsFromRadiusSearch and generateCRNeighborListsFromKNNSearch, you don't really need d_neighbor_lists. In KNN, you don't need to do Kokkos::deep_copy(device_execution_space(), d_neighbor_lists, neighbor_lists); in the beginning, as it's going to be overwritten anyway.

But this are small potatoes.

[aprokop]

@kuberry Just a quick question: do you plan to use ArborX path for any real applications? Or is the plan right now to simply have it in if a need arises? I would be really interested to see the benefits if its the former, and see what can be improved.

[kuberry]

@kuberry Just a quick question: do you plan to use ArborX path for any real applications? Or is the plan right now to simply have it in if a need arises? I would be really interested to see the benefits if its the former, and see what can be improved.

Compadre is a Trilinos package as well, so it is hard to say who all of the external users are. It is used in a number of projects at Sandia. Many of them that relate to circuit simulation are calling it in serial, however (query a single current for a voltage, query a single current for a voltage, ....).

[aprokop]

Is there any desire to resurrect this effort? We added a lot of new feature to ArborX since we last talked, including multi-dimensional points with user-specified precision.